feat: generation fact log — after-image commit records, dual-written at every commit point

Every committed generation now also appends a FACT — an after-image
commit record (what each touched entity/relationship became, or a
body-less tombstone for a removal) — to an append-only, crc32c-framed
segment log under _generations/facts/. The before-image history and the
canonical tree remain authoritative; the fact log gives consumers ONE
sequential, self-verifying stream (index heals, incremental replays)
in place of a per-entity directory walk.

- Wire format: positional msgpack facts [generation, timestamp, ops,
  meta, blobHashes]; op = [kind u8, id bin16, record | nil tombstone];
  32-byte segment header (magic, formatVersion, firstGeneration,
  zeroed+verified reserved); length+crc32c frame per fact; zero-padded
  segment names so lexicographic order == generation order; JSON
  manifest with an atomic rename flip, manifest-first rotation.
- Commit protocol: facts append+fsync BEFORE the commit point inside
  the existing durability window, so a crash can only leave the log
  AHEAD of committed truth — open() truncates back (torn tails detected
  by CRC). Absent generation = never committed; a scan can never see an
  uncommitted fact. transact() facts are durable-on-return; single-op
  facts ride the group-commit flush exactly like buffered history. A
  fact-append failure fails the write, loudly — a silent gap would be a
  lie a later replay discovers.
- New public surface: brain.scanFacts() (sequential batches with heal
  telemetry: head/segments/approx up front, per-batch generation range
  + bytes + segment id, loud abort on gaps, summary cross-check) and
  brain.factSegmentPaths() (immutable sealed segments for zero-copy
  consumers; the mutable tail excluded). Exported types CommitFact,
  FactOp, FactScanBatch, FactScanHandle.
- Storage: optional binary raw-byte primitives (appendRawBytes,
  readRawBytes, writeRawBytes, rawByteSize) on StorageAdapter —
  feature-detected; filesystem + memory adapters implement them; an
  adapter without them hosts no fact log. Fact segments are byte-copied
  (never hard-linked) into snapshots. The _generations/facts/ namespace
  is registered as a protected family (rebuildable: false): no sweeper
  or GC may delete under it.
- New crc32c (Castagnoli) utility with RFC known-answer tests.

src/utils/crc32c.ts

@@ -0,0 +1,43 @@
+/**
+ * @module utils/crc32c
+ * @description CRC-32C (Castagnoli, polynomial 0x1EDC6F41, reflected 0x82F63B78)
+ * — the storage-industry frame checksum (ext4, iSCSI, SCTP, LSM segment files).
+ * Used to frame generation-fact segments: every appended record carries the
+ * CRC-32C of its payload, so a torn tail (crash mid-append) or bit rot is
+ * DETECTED at scan time and never silently read as data.
+ *
+ * Table-driven, dependency-free reference implementation. Native providers may
+ * substitute a hardware-accelerated (SSE4.2 / ARMv8 CRC) implementation — the
+ * polynomial is the contract, byte-identical results required.
+ */
+
+/** The 256-entry lookup table for the reflected CRC-32C polynomial. */
+const TABLE: Uint32Array = (() => {
+  const table = new Uint32Array(256)
+  for (let n = 0; n < 256; n++) {
+    let c = n
+    for (let k = 0; k < 8; k++) {
+      c = c & 1 ? 0x82f63b78 ^ (c >>> 1) : c >>> 1
+    }
+    table[n] = c >>> 0
+  }
+  return table
+})()
+
+/**
+ * Compute the CRC-32C checksum of a byte buffer.
+ *
+ * Known-answer vectors (RFC 3720 appendix / the standard test suite):
+ * - ASCII "123456789" → 0xE3069283
+ * - 32 zero bytes → 0x8A9136AA
+ *
+ * @param bytes - The payload to checksum.
+ * @returns The CRC-32C as an unsigned 32-bit integer.
+ */
+export function crc32c(bytes: Uint8Array): number {
+  let crc = 0xffffffff
+  for (let i = 0; i < bytes.length; i++) {
+    crc = TABLE[(crc ^ bytes[i]) & 0xff] ^ (crc >>> 8)
+  }
+  return (crc ^ 0xffffffff) >>> 0
+}