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/errors/brainyError.ts

@@ -15,6 +15,8 @@ export type BrainyErrorType =
   | 'GRAPH_INDEX_NOT_READY'
   | 'METADATA_INDEX_NOT_READY'
   | 'VECTOR_INDEX_NOT_READY'
+  | 'PROTECTED_ARTIFACT'
+  | 'DERIVED_ARTIFACT_MISSING'
   | 'MIGRATION_IN_PROGRESS'
 
 /**
@@ -307,6 +309,60 @@ export class VectorIndexNotReadyError extends BrainyError {
     }
 }
 
+/**
+ * Thrown when a delete (`deleteBinaryBlob` / `removeRawPrefix`) would remove a
+ * blob that is a declared member of a protected derived-index FAMILY (ADR-004 §7
+ * registered-blob contract). Declared derived artifacts are undeletable through
+ * the storage layer — this makes an in-process GC / sweeper INCAPABLE of removing
+ * a load-bearing index file (the lost-`main.dkann` class). Intentional retirement
+ * is the explicit `unregisterDerivedFamily(name)` step, then the delete.
+ */
+export class ProtectedArtifactError extends BrainyError {
+    /** The blob key the delete targeted. */
+    public readonly key: string
+    /** The protected family the key belongs to. */
+    public readonly family: string
+    constructor(key: string, family: string) {
+        super(
+            `Refused to delete '${key}': it is a declared member of the protected ` +
+                `derived-index family '${family}'. Declared derived artifacts are undeletable ` +
+                `through the storage layer (COLD ≠ DEAD) — unregisterDerivedFamily('${family}') ` +
+                `first if retirement is intentional.`,
+            'PROTECTED_ARTIFACT',
+            false
+        )
+        this.name = 'ProtectedArtifactError'
+        this.key = key
+        this.family = family
+    }
+}
+
+/**
+ * Raised (loudly) when a declared derived-index family is missing one or more of
+ * its members on open — i.e. a load-bearing blob was deleted OUTSIDE the write
+ * path (an external sweeper the in-process refusal cannot stop). The index must
+ * be rebuilt from canonical; "healthy-while-broken" is impossible because the
+ * missing member is named, not silently tolerated.
+ */
+export class DerivedArtifactMissingError extends BrainyError {
+    /** The family with missing members. */
+    public readonly family: string
+    /** The member keys that are absent. */
+    public readonly missing: string[]
+    constructor(family: string, missing: string[]) {
+        super(
+            `Derived-index family '${family}' is missing ${missing.length} declared ` +
+                `member(s) on open (${missing.join(', ')}) — deleted outside the write path. ` +
+                `The index must be rebuilt from canonical.`,
+            'DERIVED_ARTIFACT_MISSING',
+            false
+        )
+        this.name = 'DerivedArtifactMissingError'
+        this.family = family
+        this.missing = missing
+    }
+}
+
 /**
  * Thrown when a data-plane read or write is issued against a brain that is
  * running its one-time, automatic 7.x → 8.0 on-disk upgrade — the coordinated