metadata schema validation

docs/asset-registry-implementation.md

@@ -33,6 +33,8 @@ and SQLite repositories are adapters behind those ports.
 - Stable `KnowledgeAsset` creation with explicit source references.
 - Separate source, normalized, and derived `AssetRepresentation` records.
 - `MetadataRecord` persistence with inferred/confirmed semantics preserved.
+- Custom metadata schema primitives with structured validation issues.
+- Metadata schema validation before asset create and metadata update writes.
 - Actor and `OperationContext` required for material mutations.
 - Policy gateway authorization before asset mutations.
 - Fail-closed policy denial through `AuthorizationError`.
@@ -70,7 +72,8 @@ idempotency key.
 
 ## Not Yet Implemented
 
-- Full custom metadata schema validation.
+- Schema registry persistence and policy-assigned schema selection.
+- Standard metadata filtering beyond lifecycle and asset type.
 - Policy assignment storage and enterprise policy adapters.
 - Conflict detection beyond version-sequence uniqueness.
 - Restore and supersession service operations.
@@ -91,4 +94,5 @@ These remain in scope for later `KONT-WP-0005` tasks or adjacent workplans.
 - idempotent asset creation and conflicting idempotency-key reuse,
 - relationship creation with source-asset versioning and audit,
 - SQLite reload preserving context entities, relationships, and idempotency
-  records.
+  records,
+- custom metadata schema validation before registry writes.