feat: implement IMetadataService contract in MetadataManager

- MetadataManager now implements full IMetadataService contract
- Core CRUD: register/get/list/unregister/exists/listNames/getObject/listObjects
- Query: query with filtering, sorting, pagination
- Bulk: bulkRegister/bulkUnregister
- Overlay: getOverlay/saveOverlay/removeOverlay/getEffective
- Watch: watchService returning MetadataWatchHandle
- Import/Export: exportMetadata/importMetadata with conflict resolution
- Validation: validate method
- Type Registry: getRegisteredTypes/getTypeInfo using DEFAULT_METADATA_TYPE_REGISTRY
- Dependency Tracking: getDependencies/getDependents/addDependency
- MetadataPlugin updated to use spec's type registry and load order
- Updated exports with IMetadataService and kernel types
- 52 new tests for IMetadataService contract, all passing (97 total)

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

packages/metadata/src/index.ts

@@ -3,7 +3,8 @@
 /**
  * @objectstack/metadata
  * 
- * Metadata loading, saving, and persistence for ObjectStack
+ * Metadata loading, saving, and persistence for ObjectStack.
+ * Implements the IMetadataService contract from @objectstack/spec.
  */
 
 // Main Manager
@@ -39,3 +40,25 @@ export type {
   MetadataLoaderContract,
   MetadataManagerConfig,
 } from '@objectstack/spec/system';
+
+// Re-export IMetadataService contract
+export type {
+  IMetadataService,
+  MetadataWatchCallback,
+  MetadataWatchHandle,
+  MetadataTypeInfo,
+  MetadataImportResult,
+} from '@objectstack/spec/contracts';
+
+// Re-export kernel types for plugin protocol
+export type {
+  MetadataType,
+  MetadataTypeRegistryEntry,
+  MetadataPluginConfig,
+  MetadataPluginManifest,
+  MetadataQuery,
+  MetadataQueryResult,
+  MetadataValidationResult,
+  MetadataBulkResult,
+  MetadataDependency,
+} from '@objectstack/spec/kernel';