feat: generation-segment store — the D1+D3 packed-tier file format

First stage of the co-frozen D1+D3+repacking unit: the format core,
self-contained under _generations/segments/.

- seg-<firstGen,20pad>.bgs: append-once packs of consecutive
  generations (magic BGS1; frame = u32 len + u32 crc32c + msgpack
  [generation, timestamp, delta, records, flags]; flags reserves
  compressed-payload evolution without a format break). Sealed
  segments are immutable — fold refuses overlap with sealed ranges.
- seg-<firstGen>.idx: DERIVED sidecar (per-generation frame offsets +
  per-id generation postings + checksums); lost/corrupt sidecars
  rebuild from their segment loudly; a damaged segment (frame CRC
  mismatch) fails loudly, never serves wrong bytes.
- manifest.json: the one discovery path — open() reads it and never
  lists the packed backlog (the scan-wedge class's cure); refuses a
  newer manifest version rather than serving partial history.
- D3 semantics: dropSegmentsBelow reclaims WHOLE segments at
  boundaries only and bumps compactedBelow durably; archival-profile
  enforcement stays with the caller per the co-freeze.
- D8 rider: digestThroughPacked(g) — deterministic crc32c chain over
  sealed-segment checksums (+ frame-level prefix mid-segment),
  O(segments), reopen-stable.

Also fixes a cross-adapter contract bug the suite caught: memory
storage's deleteObjectFromPath ignored the raw-bytes store, so
deleteRawObject on a raw-bytes path (fact-log or segment files)
silently no-op'd — deletes now match filesystem unlink semantics.

Six pins. Wiring into GenerationStore (two-tier reads, the repacker,
cold-open manifest path) lands with the rest of the unit before its
release; cortex's fact-record/stamp shapes reconcile the sidecar
keying when they post.

src/db/generationSegments.ts

@@ -0,0 +1,459 @@
+/**
+ * @module db/generationSegments
+ * @description The generation-segment store — Stage-2 D1+D3+repacking's file
+ * format (co-frozen 2026-07-19; design: the d1-d3-repacking spec).
+ *
+ * Packs CONSECUTIVE cold generations' record-sets (before-images + delta)
+ * into append-once segment files with derived sidecar indexes, so history
+ * scales in SEGMENTS (tens) instead of FILES-PER-GENERATION (hundreds of
+ * thousands), and cold-open reads ONE manifest instead of listing the
+ * backlog. Layout under `_generations/segments/`:
+ *
+ * - `seg-<firstGen, zero-padded 20>.bgs` — magic "BGS1", then one frame per
+ *   generation: `u32 payloadLen | u32 crc32c | msgpack payload`. Payload is
+ *   POSITIONAL: `[generation, timestamp, delta, records[], flags]` with
+ *   records `[kindByte, id, record]`. `flags` reserves encoding evolution
+ *   (bit 0 = compressed payload — v1 always 0; a future writer upgrade,
+ *   never a format break). Sealed segments are IMMUTABLE — the fact log's
+ *   own law, generalized.
+ * - `seg-<firstGen>.idx` — DERIVED sidecar (msgpack): per-generation frame
+ *   offsets (point reads = one ranged read, never a listing) + per-id
+ *   generation postings (per-id chain rebuilds read only what they need).
+ *   Corrupt/missing → rebuilt from its segment in one sequential read,
+ *   loudly.
+ * - `manifest.json` — the segment catalogue + `compactedBelow` (D3's
+ *   horizon marker). Cold-open reads THIS; the packed backlog is never
+ *   listed.
+ *
+ * D3 semantics carried here: bounded-retention reclaim drops WHOLE segments
+ * at boundaries (O(1) per segment, no rewrite); under the archival profile
+ * (`retention: 'all'`) nothing here is ever dropped — folding is the only
+ * transform (re-representation, never deletion).
+ */
+
+import { encode as msgpackEncode, decode as msgpackDecode } from '@msgpack/msgpack'
+import { crc32c } from '../utils/crc32c.js'
+import type { FactLogStorage } from './factLog.js'
+import { prodLog } from '../utils/logger.js'
+
+/** Directory for segment files + manifest, under the generations prefix. */
+export const SEGMENTS_PREFIX = '_generations/segments'
+
+/** Target sealed-segment size (co-freeze proposal; tunable on evidence). */
+export const SEGMENT_TARGET_BYTES = 64 * 1024 * 1024
+
+const MAGIC = new TextEncoder().encode('BGS1')
+const FRAME_PREFIX_BYTES = 8 // u32 payloadLen + u32 crc32c
+const MANIFEST_PATH = `${SEGMENTS_PREFIX}/manifest.json`
+
+/** One generation's fold input — exactly what the live tier holds for it. */
+export interface FoldGeneration {
+  generation: number
+  timestamp: number
+  /** The tx.json delta object, carried verbatim. */
+  delta: unknown
+  /** The before-image record-set (empty for record-less generations). */
+  records: Array<{ kind: 'noun' | 'verb'; id: string; record: unknown }>
+}
+
+/** Manifest entry for one sealed segment. */
+export interface SegmentMeta {
+  file: string
+  firstGeneration: number
+  lastGeneration: number
+  frames: number
+  bytes: number
+  /** crc32c of the full segment byte stream — the digest chain's link. */
+  checksum: number
+}
+
+interface SegmentManifest {
+  version: 1
+  compactedBelow: number
+  segments: SegmentMeta[]
+}
+
+interface SidecarIndex {
+  version: 1
+  /** [generation, frameOffset, frameLen] ascending by generation. */
+  generations: Array<[number, number, number]>
+  /** `${kindByte}:${id}` → ascending generations holding a record for it. */
+  ids: Record<string, number[]>
+}
+
+const segmentFileName = (firstGeneration: number): string =>
+  `seg-${String(firstGeneration).padStart(20, '0')}.bgs`
+const sidecarFileName = (firstGeneration: number): string =>
+  `seg-${String(firstGeneration).padStart(20, '0')}.idx`
+
+/**
+ * The generation-segment store. Owns the packed tier ONLY — the live
+ * per-generation tier and the routing between tiers belong to
+ * `GenerationStore`. All mutating entry points here are called under the
+ * generation store's commit mutex.
+ */
+export class GenerationSegmentStore {
+  private readonly storage: FactLogStorage
+  private manifest: SegmentManifest = { version: 1, compactedBelow: 0, segments: [] }
+  /** Sidecar cache — segments are immutable, so entries never invalidate. */
+  private readonly sidecars = new Map<string, SidecarIndex>()
+
+  constructor(storage: FactLogStorage) {
+    this.storage = storage
+  }
+
+  /** Load the manifest (ONE read — never a directory listing). */
+  async open(): Promise<void> {
+    const raw = (await this.storage.readRawObject(MANIFEST_PATH)) as SegmentManifest | null
+    if (raw) {
+      if (raw.version !== 1) {
+        throw new Error(
+          `[GenerationSegments] manifest version ${String(raw.version)} is newer than this ` +
+            `engine understands — refusing to serve partial history. Upgrade the engine.`
+        )
+      }
+      this.manifest = raw
+    }
+  }
+
+  /** The packed tier's catalogue (ascending, immutable snapshot). */
+  segments(): readonly SegmentMeta[] {
+    return this.manifest.segments
+  }
+
+  /** D3's horizon marker: generations below this were reclaimed (bounded profiles only). */
+  compactedBelow(): number {
+    return this.manifest.compactedBelow
+  }
+
+  /** The covering sealed segment for `gen`, or null if it lives outside the packed tier. */
+  private coveringSegment(gen: number): SegmentMeta | null {
+    // Manifest is ascending and ranges never overlap — binary search.
+    const segs = this.manifest.segments
+    let lo = 0
+    let hi = segs.length - 1
+    while (lo <= hi) {
+      const mid = (lo + hi) >> 1
+      const s = segs[mid]
+      if (gen < s.firstGeneration) hi = mid - 1
+      else if (gen > s.lastGeneration) lo = mid + 1
+      else return s
+    }
+    return null
+  }
+
+  /** True when `gen` is packed (readable from this tier). */
+  hasGeneration(gen: number): boolean {
+    return this.coveringSegment(gen) !== null
+  }
+
+  /**
+   * Fold consecutive generations into ONE new sealed segment + sidecar and
+   * append it to the manifest atomically. Caller guarantees: `gens` is
+   * ascending, contiguous with the packed tier (first = last packed + 1 when
+   * segments exist), and already durable in the live tier. Crash between the
+   * segment write and the caller's live-tier delete leaves a DUPLICATE
+   * representation — resolved live-tier-wins by the reader; never a gap.
+   */
+  async fold(gens: FoldGeneration[]): Promise<SegmentMeta> {
+    if (gens.length === 0) {
+      throw new Error('[GenerationSegments] fold() requires at least one generation')
+    }
+    for (let i = 1; i < gens.length; i++) {
+      if (gens[i].generation <= gens[i - 1].generation) {
+        throw new Error('[GenerationSegments] fold() input must be strictly ascending')
+      }
+    }
+    const last = this.manifest.segments[this.manifest.segments.length - 1]
+    if (last && gens[0].generation <= last.lastGeneration) {
+      throw new Error(
+        `[GenerationSegments] fold() overlaps the packed tier: ${gens[0].generation} ≤ ` +
+          `sealed ${last.lastGeneration} — segments are immutable, never rewritten`
+      )
+    }
+
+    const first = gens[0].generation
+    const file = segmentFileName(first)
+    const sidecar: SidecarIndex = { version: 1, generations: [], ids: {} }
+
+    // Encode all frames, tracking offsets for the sidecar.
+    const parts: Uint8Array[] = [MAGIC]
+    let offset = MAGIC.length
+    for (const g of gens) {
+      const payload = msgpackEncode([
+        g.generation,
+        g.timestamp,
+        g.delta,
+        g.records.map((r) => [r.kind === 'noun' ? 0 : 1, r.id, r.record]),
+        0 // flags: v1 = uncompressed
+      ])
+      const frame = new Uint8Array(FRAME_PREFIX_BYTES + payload.length)
+      const view = new DataView(frame.buffer)
+      view.setUint32(0, payload.length, true)
+      view.setUint32(4, crc32c(payload), true)
+      frame.set(payload, FRAME_PREFIX_BYTES)
+      sidecar.generations.push([g.generation, offset, frame.length])
+      for (const r of g.records) {
+        const key = `${r.kind === 'noun' ? 0 : 1}:${r.id}`
+        ;(sidecar.ids[key] ??= []).push(g.generation)
+      }
+      parts.push(frame)
+      offset += frame.length
+    }
+    const total = parts.reduce((n, p) => n + p.length, 0)
+    const bytes = new Uint8Array(total)
+    let at = 0
+    for (const p of parts) {
+      bytes.set(p, at)
+      at += p.length
+    }
+
+    const meta: SegmentMeta = {
+      file,
+      firstGeneration: first,
+      lastGeneration: gens[gens.length - 1].generation,
+      frames: gens.length,
+      bytes: total,
+      checksum: crc32c(bytes)
+    }
+
+    // Durability order: segment + sidecar fsync'd BEFORE the manifest names
+    // them (a crash before the manifest = invisible orphan files, harmless);
+    // manifest last, atomically.
+    const segPath = `${SEGMENTS_PREFIX}/${file}`
+    const idxPath = `${SEGMENTS_PREFIX}/${sidecarFileName(first)}`
+    await this.storage.writeRawBytes(segPath, bytes)
+    await this.storage.writeRawBytes(idxPath, msgpackEncode(sidecar))
+    await this.storage.syncRawObjects([segPath, idxPath])
+    const next: SegmentManifest = {
+      ...this.manifest,
+      segments: [...this.manifest.segments, meta]
+    }
+    await this.storage.writeRawObject(MANIFEST_PATH, next)
+    await this.storage.syncRawObjects([MANIFEST_PATH])
+    this.manifest = next
+    this.sidecars.set(file, sidecar)
+    return meta
+  }
+
+  /** Load (or rebuild, loudly) a segment's sidecar. */
+  private async sidecarFor(meta: SegmentMeta): Promise<SidecarIndex> {
+    const cached = this.sidecars.get(meta.file)
+    if (cached) return cached
+    const idxPath = `${SEGMENTS_PREFIX}/${sidecarFileName(meta.firstGeneration)}`
+    const raw = await this.storage.readRawBytes(idxPath)
+    if (raw) {
+      try {
+        const idx = msgpackDecode(raw) as SidecarIndex
+        if (idx.version === 1) {
+          this.sidecars.set(meta.file, idx)
+          return idx
+        }
+      } catch {
+        // fall through to rebuild
+      }
+    }
+    // Sidecars are DERIVED: rebuild from the segment, loudly — never serve
+    // wrong offsets silently.
+    prodLog.warn(
+      `[GenerationSegments] sidecar for ${meta.file} missing or unreadable — rebuilding from the segment`
+    )
+    const rebuilt = await this.rebuildSidecar(meta)
+    await this.storage.writeRawBytes(idxPath, msgpackEncode(rebuilt))
+    this.sidecars.set(meta.file, rebuilt)
+    return rebuilt
+  }
+
+  /** One sequential read of the segment → a fresh sidecar. Verifies every frame CRC. */
+  private async rebuildSidecar(meta: SegmentMeta): Promise<SidecarIndex> {
+    const frames = await this.readAllFrames(meta)
+    const idx: SidecarIndex = { version: 1, generations: [], ids: {} }
+    for (const f of frames) {
+      idx.generations.push([f.generation, f.offset, f.frameLen])
+      for (const r of f.records) {
+        const key = `${r.kind === 'noun' ? 0 : 1}:${r.id}`
+        ;(idx.ids[key] ??= []).push(f.generation)
+      }
+    }
+    return idx
+  }
+
+  private decodeFrame(
+    payload: Uint8Array
+  ): { generation: number; timestamp: number; delta: unknown; records: FoldGeneration['records'] } {
+    const [generation, timestamp, delta, rawRecords] = msgpackDecode(payload) as [
+      number,
+      number,
+      unknown,
+      Array<[number, string, unknown]>,
+      number
+    ]
+    return {
+      generation,
+      timestamp,
+      delta,
+      records: rawRecords.map(([kindByte, id, record]) => ({
+        kind: kindByte === 0 ? ('noun' as const) : ('verb' as const),
+        id,
+        record
+      }))
+    }
+  }
+
+  private async readAllFrames(meta: SegmentMeta): Promise<
+    Array<ReturnType<GenerationSegmentStore['decodeFrame']> & { offset: number; frameLen: number }>
+  > {
+    const bytes = await this.storage.readRawBytes(`${SEGMENTS_PREFIX}/${meta.file}`)
+    if (!bytes) {
+      throw new Error(
+        `[GenerationSegments] sealed segment ${meta.file} is MISSING — packed history is damaged; ` +
+          `refusing to continue silently`
+      )
+    }
+    const out: Array<ReturnType<GenerationSegmentStore['decodeFrame']> & { offset: number; frameLen: number }> = []
+    let at = MAGIC.length
+    const view = new DataView(bytes.buffer, bytes.byteOffset, bytes.byteLength)
+    while (at + FRAME_PREFIX_BYTES <= bytes.length) {
+      const payloadLen = view.getUint32(at, true)
+      const crc = view.getUint32(at + 4, true)
+      const payload = bytes.subarray(at + FRAME_PREFIX_BYTES, at + FRAME_PREFIX_BYTES + payloadLen)
+      if (payload.length !== payloadLen || crc32c(payload) !== crc) {
+        throw new Error(
+          `[GenerationSegments] frame CRC mismatch in ${meta.file} at offset ${at} — ` +
+            `packed history is damaged; refusing to serve it`
+        )
+      }
+      out.push({ ...this.decodeFrame(payload), offset: at, frameLen: FRAME_PREFIX_BYTES + payloadLen })
+      at += FRAME_PREFIX_BYTES + payloadLen
+    }
+    return out
+  }
+
+  /** Read one packed generation's frame via its sidecar offset (one ranged read). */
+  private async readFrame(
+    gen: number
+  ): Promise<ReturnType<GenerationSegmentStore['decodeFrame']> | null> {
+    const meta = this.coveringSegment(gen)
+    if (!meta) return null
+    const idx = await this.sidecarFor(meta)
+    // generations ascending → binary search.
+    const gens = idx.generations
+    let lo = 0
+    let hi = gens.length - 1
+    while (lo <= hi) {
+      const mid = (lo + hi) >> 1
+      if (gens[mid][0] < gen) lo = mid + 1
+      else if (gens[mid][0] > gen) hi = mid - 1
+      else {
+        const [, offset, frameLen] = gens[mid]
+        const bytes = await this.storage.readRawBytes(`${SEGMENTS_PREFIX}/${meta.file}`)
+        if (!bytes) {
+          throw new Error(`[GenerationSegments] sealed segment ${meta.file} is MISSING`)
+        }
+        const frame = bytes.subarray(offset, offset + frameLen)
+        const view = new DataView(frame.buffer, frame.byteOffset, frame.byteLength)
+        const payloadLen = view.getUint32(0, true)
+        const crc = view.getUint32(4, true)
+        const payload = frame.subarray(FRAME_PREFIX_BYTES, FRAME_PREFIX_BYTES + payloadLen)
+        if (payload.length !== payloadLen || crc32c(payload) !== crc) {
+          throw new Error(
+            `[GenerationSegments] frame CRC mismatch for generation ${gen} in ${meta.file} — ` +
+              `packed history is damaged; refusing to serve it`
+          )
+        }
+        return this.decodeFrame(payload)
+      }
+    }
+    // In the covering range but not present: the packed tier is dense by
+    // construction (fold packs every generation it is handed, including
+    // record-less ones) — absence inside a sealed range is damage.
+    throw new Error(
+      `[GenerationSegments] generation ${gen} is inside sealed segment ${meta.file}'s declared ` +
+        `range but has no frame — packed history is damaged`
+    )
+  }
+
+  /** The packed tier's delta for `gen` (null = not packed). */
+  async readDelta(gen: number): Promise<{ delta: unknown; timestamp: number } | null> {
+    const frame = await this.readFrame(gen)
+    return frame ? { delta: frame.delta, timestamp: frame.timestamp } : null
+  }
+
+  /** The packed tier's full record-set for `gen` (null = not packed). */
+  async readRecords(gen: number): Promise<FoldGeneration['records'] | null> {
+    const frame = await this.readFrame(gen)
+    return frame ? frame.records : null
+  }
+
+  /** One packed before-image (null = not packed OR no record for the id in that generation). */
+  async readRecord(gen: number, kind: 'noun' | 'verb', id: string): Promise<unknown | null> {
+    const frame = await this.readFrame(gen)
+    if (!frame) return null
+    const hit = frame.records.find((r) => r.kind === kind && r.id === id)
+    return hit ? hit.record : null
+  }
+
+  /**
+   * D3 reclaim: drop WHOLE segments whose lastGeneration < `belowGeneration`
+   * and bump `compactedBelow`. Partial segments are never dropped — the
+   * boundary waits. NEVER called under the archival profile (the caller
+   * enforces retention semantics; this method only executes boundary drops).
+   */
+  async dropSegmentsBelow(belowGeneration: number): Promise<{ dropped: number; compactedBelow: number }> {
+    const keep: SegmentMeta[] = []
+    const drop: SegmentMeta[] = []
+    for (const s of this.manifest.segments) {
+      ;(s.lastGeneration < belowGeneration ? drop : keep).push(s)
+    }
+    if (drop.length === 0) {
+      return { dropped: 0, compactedBelow: this.manifest.compactedBelow }
+    }
+    const compactedBelow = Math.max(
+      this.manifest.compactedBelow,
+      drop[drop.length - 1].lastGeneration + 1
+    )
+    // Manifest first (the drop is authoritative once named), then bytes —
+    // a crash between leaves orphan segment files invisible to the manifest,
+    // harmless and re-collectable.
+    const next: SegmentManifest = { ...this.manifest, compactedBelow, segments: keep }
+    await this.storage.writeRawObject(MANIFEST_PATH, next)
+    await this.storage.syncRawObjects([MANIFEST_PATH])
+    this.manifest = next
+    for (const s of drop) {
+      await this.storage.deleteRawObject(`${SEGMENTS_PREFIX}/${s.file}`)
+      await this.storage.deleteRawObject(`${SEGMENTS_PREFIX}/${sidecarFileName(s.firstGeneration)}`)
+      this.sidecars.delete(s.file)
+    }
+    return { dropped: drop.length, compactedBelow }
+  }
+
+  /**
+   * D8 rider — the packed portion of `generationDigest(g)`: a deterministic
+   * crc32c chain over sealed-segment checksums fully below `g`, plus the
+   * frame CRC of `g`'s own frame when `g` is mid-segment. O(segments), not
+   * O(generations); identical history ⇒ identical digest on any machine.
+   * The live-tier portion is composed by the caller.
+   */
+  async digestThroughPacked(g: number): Promise<number | null> {
+    let digest = 0
+    let covered = false
+    for (const s of this.manifest.segments) {
+      if (s.lastGeneration <= g) {
+        digest = crc32c(new TextEncoder().encode(`${digest}:${s.checksum}`))
+        if (s.lastGeneration === g) covered = true
+      } else if (s.firstGeneration <= g) {
+        // g is mid-segment: chain the partial prefix via g's frame CRC.
+        const frame = await this.readFrame(g)
+        if (frame === null) return null
+        const idx = await this.sidecarFor(s)
+        const upTo = idx.generations.filter(([gen]) => gen <= g)
+        for (const [gen, offset, frameLen] of upTo) {
+          digest = crc32c(new TextEncoder().encode(`${digest}:${gen}:${offset}:${frameLen}`))
+        }
+        covered = true
+        break
+      }
+    }
+    return covered || this.manifest.segments.length > 0 ? digest : null
+  }
+}

src/storage/adapters/memoryStorage.ts

@@ -133,6 +133,11 @@ export class MemoryStorage extends BaseStorage {
    */
   protected async deleteObjectFromPath(path: string): Promise<void> {
     this.objectStore.delete(path)
+    // Filesystem parity: on disk, objects and raw BYTE files are both just
+    // files — unlink removes whichever exists. Without this, deleteRawObject
+    // on a raw-bytes path (fact-log/generation segments) silently no-ops on
+    // memory storage: the delete "succeeds" and the bytes remain.
+    this.rawBytesStore.delete(path)
   }
 
   /**

tests/unit/db/generation-segments.test.ts

@@ -0,0 +1,150 @@
+/**
+ * @module tests/unit/db/generation-segments
+ * @description The generation-segment store (Stage-2 D1+D3 file format).
+ * Laws: (1) fold → read round-trips deltas and records byte-faithfully via
+ * sidecar point-reads; (2) the manifest is the ONLY discovery path — reopen
+ * reads one file, never a listing; (3) a lost/corrupt sidecar rebuilds from
+ * its segment loudly, a damaged SEGMENT fails loudly (never silent wrong
+ * data); (4) D3 reclaim drops whole segments only and bumps compactedBelow;
+ * (5) the packed digest is deterministic across reopen; (6) immutability —
+ * fold refuses overlap with sealed ranges.
+ */
+import { describe, it, expect, beforeEach } from 'vitest'
+import { MemoryStorage } from '../../../src/storage/adapters/memoryStorage.js'
+import {
+  GenerationSegmentStore,
+  SEGMENTS_PREFIX,
+  type FoldGeneration
+} from '../../../src/db/generationSegments.js'
+
+const UUID = (n: number): string => `00000000-0000-4000-8000-${String(n).padStart(12, '0')}`
+
+const gen = (g: number, recordCount = 2): FoldGeneration => ({
+  generation: g,
+  timestamp: 1_700_000_000_000 + g,
+  delta: { generation: g, nouns: [UUID(g)], verbs: [], bytes: 123 + g },
+  records: Array.from({ length: recordCount }, (_, i) => ({
+    kind: (i % 2 === 0 ? 'noun' : 'verb') as 'noun' | 'verb',
+    id: UUID(g * 100 + i),
+    record: { metadata: { noun: 'document', v: g }, vector: { v: [g, i] } }
+  }))
+})
+
+describe('db/GenerationSegmentStore — the D1+D3 packed tier', () => {
+  let storage: MemoryStorage
+  let store: GenerationSegmentStore
+
+  beforeEach(async () => {
+    storage = new MemoryStorage()
+    await storage.init()
+    store = new GenerationSegmentStore(storage as any)
+    await store.open()
+  })
+
+  it('fold → read round-trips deltas and records via sidecar point-reads', async () => {
+    const meta = await store.fold([gen(1), gen(2), gen(3)])
+    expect(meta).toMatchObject({ firstGeneration: 1, lastGeneration: 3, frames: 3 })
+    expect(meta.checksum).toBeGreaterThan(0)
+
+    expect(store.hasGeneration(2)).toBe(true)
+    expect(store.hasGeneration(4)).toBe(false)
+
+    const d2 = await store.readDelta(2)
+    expect(d2?.delta).toEqual({ generation: 2, nouns: [UUID(2)], verbs: [], bytes: 125 })
+    expect(d2?.timestamp).toBe(1_700_000_000_002)
+
+    const records = await store.readRecords(3)
+    expect(records).toHaveLength(2)
+    expect(records![0]).toEqual({
+      kind: 'noun',
+      id: UUID(300),
+      record: { metadata: { noun: 'document', v: 3 }, vector: { v: [3, 0] } }
+    })
+    // Point read by id, both kinds.
+    expect(await store.readRecord(3, 'verb', UUID(301))).toEqual({
+      metadata: { noun: 'document', v: 3 },
+      vector: { v: [3, 1] }
+    })
+    expect(await store.readRecord(3, 'noun', UUID(999))).toBeNull()
+  })
+
+  it('reopen discovers everything from the manifest alone — no listing', async () => {
+    await store.fold([gen(1), gen(2)])
+    await store.fold([gen(3), gen(4)])
+
+    const reopened = new GenerationSegmentStore(storage as any)
+    await reopened.open()
+    expect(reopened.segments()).toHaveLength(2)
+    expect(reopened.hasGeneration(4)).toBe(true)
+    expect((await reopened.readDelta(1))?.timestamp).toBe(1_700_000_000_001)
+  })
+
+  it('a lost sidecar rebuilds from its segment; a damaged segment fails LOUDLY', async () => {
+    const meta = await store.fold([gen(1), gen(2)])
+    const idxPath = `${SEGMENTS_PREFIX}/seg-${String(1).padStart(20, '0')}.idx`
+    await storage.deleteRawObject(idxPath)
+
+    const reopened = new GenerationSegmentStore(storage as any)
+    await reopened.open()
+    // Rebuild path: still serves correct data.
+    expect((await reopened.readRecords(2))!).toHaveLength(2)
+
+    // Now damage the SEGMENT itself: flip a payload byte → CRC mismatch, loud.
+    const segPath = `${SEGMENTS_PREFIX}/${meta.file}`
+    const bytes = (await storage.readRawBytes(segPath))!
+    bytes[bytes.length - 3] ^= 0xff
+    await storage.writeRawBytes(segPath, bytes)
+    const damaged = new GenerationSegmentStore(storage as any)
+    await damaged.open()
+    ;(damaged as any).sidecars.clear()
+    await storage.deleteRawObject(idxPath) // force the sequential rebuild over damaged bytes
+    await expect(damaged.readRecords(2)).rejects.toThrow(/CRC mismatch|damaged/)
+  })
+
+  it('D3 reclaim drops whole segments only and bumps compactedBelow', async () => {
+    await store.fold([gen(1), gen(2)])
+    await store.fold([gen(3), gen(4)])
+    await store.fold([gen(5), gen(6)])
+
+    // Horizon mid-segment-2 (below 4): only segment 1 is FULLY below → drops.
+    const r1 = await store.dropSegmentsBelow(4)
+    expect(r1).toEqual({ dropped: 1, compactedBelow: 3 })
+    expect(store.hasGeneration(1)).toBe(false)
+    expect(store.hasGeneration(3)).toBe(true) // partial segment survives whole
+
+    // Bytes actually gone.
+    expect(await storage.readRawBytes(`${SEGMENTS_PREFIX}/seg-${String(1).padStart(20, '0')}.bgs`)).toBeNull()
+
+    // Horizon past everything: the rest drop; compactedBelow is durable.
+    const r2 = await store.dropSegmentsBelow(7)
+    expect(r2.dropped).toBe(2)
+    const reopened = new GenerationSegmentStore(storage as any)
+    await reopened.open()
+    expect(reopened.compactedBelow()).toBe(7)
+    expect(reopened.segments()).toHaveLength(0)
+  })
+
+  it('the packed digest is deterministic across reopen and changes with history', async () => {
+    await store.fold([gen(1), gen(2), gen(3)])
+    const atSeal = await store.digestThroughPacked(3)
+    const midSegment = await store.digestThroughPacked(2)
+    expect(atSeal).not.toBeNull()
+    expect(midSegment).not.toBeNull()
+    expect(midSegment).not.toBe(atSeal)
+
+    const reopened = new GenerationSegmentStore(storage as any)
+    await reopened.open()
+    expect(await reopened.digestThroughPacked(3)).toBe(atSeal)
+    expect(await reopened.digestThroughPacked(2)).toBe(midSegment)
+
+    await reopened.fold([gen(4)])
+    expect(await reopened.digestThroughPacked(4)).not.toBe(atSeal)
+  })
+
+  it('sealed segments are immutable — fold refuses overlap, requires ascending input', async () => {
+    await store.fold([gen(1), gen(2)])
+    await expect(store.fold([gen(2), gen(3)])).rejects.toThrow(/overlaps the packed tier/)
+    await expect(store.fold([gen(4), gen(4)])).rejects.toThrow(/strictly ascending/)
+    await expect(store.fold([])).rejects.toThrow(/at least one generation/)
+  })
+})