feat: registered-blob family contract — declared index blobs are undeletable (ADR-004 Pass 2)

The class-killer for the lost-main.dkann incident (ADR-004 §7). A provider can
declare a derived-index blob FAMILY (a set of members that are load-bearing
together, e.g. vector-base = main.dkann + main.slotmap + main.slotrev). Once
declared:

- deleteBinaryBlob / removeRawPrefix REFUSE to remove a declared member, throwing
  the new ProtectedArtifactError — an in-process GC / sweeper is now INCAPABLE of
  deleting a load-bearing index file (COLD != DEAD). Intentional retirement is an
  explicit unregisterDerivedFamily(name) first.
- The declaration persists to _system/derived-artifacts.json, so protection
  survives a reopen; clear() resets it with the rest of the derived footprint.
- checkDerivedFamiliesPresent() names any member missing on open (the catch for an
  EXTERNAL deleter that bypasses the in-process refusal) → rebuild from canonical.
- Transients (*.tmp.*, *.rebuild-tmp, *.rotate-tmp) are never protected; a
  namespace family protects a growing prefix (seg-*).

New StorageAdapter surface (optional): registerDerivedFamily / unregisterDerivedFamily
/ listDerivedFamilies + DerivedFamilyDeclaration; new exported errors
ProtectedArtifactError / DerivedArtifactMissingError. Enforcement is inert until a
provider declares a family (no regression). Cor declares its 6 families + does the
atomic set-swap in 3.0.15 (M2); brainy builds the contract now. 8 tests.

src/index.ts

@@ -150,7 +150,7 @@ export { EntityNotFoundError, RelationNotFoundError } from './errors/notFound.js
 
 // Base error + typed migration-lock error — thrown by any data-plane call while a
 // brain runs its one-time 7.x→8.0 upgrade; catch to answer HTTP 503 + Retry-After.
-export { BrainyError, MigrationInProgressError, GraphIndexNotReadyError, MetadataIndexNotReadyError, VectorIndexNotReadyError } from './errors/brainyError.js'
+export { BrainyError, MigrationInProgressError, GraphIndexNotReadyError, MetadataIndexNotReadyError, VectorIndexNotReadyError, ProtectedArtifactError, DerivedArtifactMissingError } from './errors/brainyError.js'
 export type { BrainyErrorType } from './errors/brainyError.js'
 
 // ============= 8.0 Db API — generational MVCC =============
@@ -301,7 +301,8 @@ import type {
   HNSWNoun,
   HNSWVerb,
   HNSWConfig,
-  StorageAdapter
+  StorageAdapter,
+  DerivedFamilyDeclaration
 } from './coreTypes.js'
 
 // Export vector index implementation (the JS HNSW path)
@@ -319,7 +320,8 @@ export type {
   HNSWNoun,
   HNSWVerb,
   HNSWConfig,
-  StorageAdapter
+  StorageAdapter,
+  DerivedFamilyDeclaration
 }
 
 // Export graph types