feat: provider access to the fact log + shared stamp verifier via internals

Three additive surfaces for native index providers, which hold only
`storage` and must never construct their own fact-log reader (the log's
open path is writer-side — it reconciles by truncating/rewriting):

- Fact-scan capability on the storage adapter (the getBinaryBlobPath
  pattern): the host brain wires a closure over its LIVE fact log at
  init; providers call storage.scanFacts()/factLogHeadGeneration()/
  factSegmentPaths() and fall back to the enumeration walk on null.
  Restore/reopen swaps the underlying log transparently.
- The family-stamp trio (readFamilyStamp/writeFamilyStamp/
  verifyFamilyStamp + types) exported from @soulcraft/brainy/internals,
  so 'one verifier' is literally one function shared with the native
  side, never two synchronized copies.
- Rollup invariants widened to number|string: content fingerprints
  (e.g. a per-tree SHA-256) are valid invariant values; strict equality
  either way, and a type mismatch reads as incoherence, never a pass.

src/db/familyStamp.ts

@@ -41,10 +41,15 @@ export interface EnumeratedMember {
   bytes: number
 }
 
-/** The stamp's verified surface, in one of the two member modes. */
+/**
+ * The stamp's verified surface, in one of the two member modes. Rollup
+ * invariant values may be numbers (counts, byte sizes) or strings (content
+ * fingerprints, e.g. a per-tree SHA-256) — the verifier compares by strict
+ * equality either way, so a type mismatch reads as incoherence, never a pass.
+ */
 export type StampMembers =
   | { mode: 'enumerated'; files: EnumeratedMember[] }
-  | { mode: 'rollup'; invariants: Record<string, number> }
+  | { mode: 'rollup'; invariants: Record<string, number | string> }
 
 /** The generalized family stamp (one shape, one verifier, both engines). */
 export interface FamilyStamp {
@@ -109,7 +114,7 @@ export async function writeFamilyStamp(
 export function verifyFamilyStamp(
   stamp: FamilyStamp | null,
   head: number,
-  actual: Record<string, number>
+  actual: Record<string, number | string>
 ): StampVerdict {
   if (stamp === null) return { state: 'absent' }
   if (stamp.sourceGeneration > head) {