feat: scanFacts liveness contract — first batch or loud failure within a documented bound

Stage-2 D1 contract item (co-frozen): a fact scan may be slow, never
silent. batches() now races its FIRST pull against
SCANFACTS_FIRST_BATCH_MS (10s, exported; test-overridable) — a wedged
or unreadably slow store produces a loud abort naming the contract
instead of a consumer hanging indistinguishably from progress (the
production shape: a heal against a generations-backlogged brain
wedged silently on the first segment read).

Only the first pull is raced: the bound is time-to-first-batch (proof
the producer is alive), not per-batch pacing, and it runs only while
a pull is pending — consumer think-time between pulls never counts
against the producer (pinned).

Three pins: wedged-store loud failure within the bound, healthy scan
untouched end-to-end, slow-consumer immunity.

src/db/factLog.ts

@@ -102,12 +102,26 @@ export interface FactScanBatch {
   segmentId: string
 }
 
+/**
+ * Liveness bound on a scan's FIRST batch (Stage-2 co-freeze, D1 contract):
+ * `batches()` must yield its first batch — or fail loudly — within this many
+ * ms of the first pull. A backlogged or damaged store may be SLOW, but it may
+ * never be SILENT: a consumer awaiting the first batch is otherwise
+ * indistinguishable from a wedge (the exact failure shape a production heal
+ * hit against a generations-backlogged brain).
+ */
+export const SCANFACTS_FIRST_BATCH_MS = 10_000
+
 /** The telemetry a scan OPEN returns (frozen shape). */
 export interface FactScanHandle {
   headGeneration: number
   segmentCount: number
   approxFactCount: number
-  /** Ordered batches; a detected gap aborts LOUDLY, never a silent skip. */
+  /**
+   * Ordered batches; a detected gap aborts LOUDLY, never a silent skip.
+   * Liveness contract: the FIRST batch resolves or rejects within
+   * {@link SCANFACTS_FIRST_BATCH_MS} of the first pull — never a silent hang.
+   */
   batches: () => AsyncGenerator<FactScanBatch>
   /** Close telemetry — the invariant cross-check, valid after iteration ends. */
   summary: () => { factsYielded: number; segmentsRead: number }
@@ -440,6 +454,8 @@ export class FactLog {
     toGeneration?: number
     kinds?: Array<'noun' | 'verb'>
     batchSize?: number
+    /** Test override for the first-batch liveness bound (default {@link SCANFACTS_FIRST_BATCH_MS}). */
+    firstBatchTimeoutMs?: number
   }): FactScanHandle {
     const from = options?.fromGeneration ?? 1
     const to = options?.toGeneration ?? this.head
@@ -514,11 +530,43 @@ export class FactLog {
       }
     }
 
+    // Liveness wrapper: the FIRST pull races the contract deadline. Only the
+    // first — the bound is time-to-first-batch (proof the producer is alive),
+    // not per-batch pacing; and it runs only while a pull is actually pending,
+    // so consumer think-time between pulls never counts against the producer.
+    const firstBatchTimeoutMs = options?.firstBatchTimeoutMs ?? SCANFACTS_FIRST_BATCH_MS
+    async function* batchesWithLiveness(this: void): AsyncGenerator<FactScanBatch> {
+      const inner = batches()
+      let timer: NodeJS.Timeout | undefined
+      try {
+        const deadline = new Promise<never>((_, reject) => {
+          timer = setTimeout(
+            () =>
+              reject(
+                new Error(
+                  `fact log: scanFacts produced no first batch within ${firstBatchTimeoutMs}ms ` +
+                    `(liveness contract) — the store is wedged or unreadably slow; aborting scan LOUDLY ` +
+                    `instead of hanging the consumer.`
+                )
+              ),
+            firstBatchTimeoutMs
+          )
+          timer.unref?.()
+        })
+        const first = await Promise.race([inner.next(), deadline])
+        if (first.done) return
+        yield first.value
+      } finally {
+        clearTimeout(timer)
+      }
+      yield* inner
+    }
+
     return {
       headGeneration: this.head,
       segmentCount: segments.length + (tailSnapshot.length > 0 ? 1 : 0),
       approxFactCount,
-      batches,
+      batches: batchesWithLiveness,
       summary: () => ({ factsYielded, segmentsRead })
     }
   }

src/index.ts

@@ -213,6 +213,7 @@ export type {
   CommitFact,
   FactOp,
   FactScanBatch,
+  SCANFACTS_FIRST_BATCH_MS,
   FactScanHandle
 } from './db/factLog.js'
 // The generalized family stamp — which source generation a projection