docs(kernel): add the MetadataWatchEvent row this PR's own fence now depends on

Restoring subscribe? to the Interface Definition fence brought in
MetadataWatchEvent, the only Metadata* type in that fence without a row in the
page's own Types Reference table. Read the shape from its actual declaration
(packages/spec/src/system/metadata-persistence.zod.ts, MetadataWatchEventSchema)
rather than inferring it from MetadataWatchCallback's differently-shaped event.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012GKcPZbMoGq7WPzKLfRBTU

Author: claude

content/docs/kernel/contracts/metadata-service.mdx

@@ -328,6 +328,7 @@ console.log(result.failed);    // failed
 | `MetadataBulkResult` | Result of `bulkRegister` / `bulkUnregister` |
 | `MetadataWatchCallback` | `(event: { type, metadataType, name, data? }) => void` |
 | `MetadataWatchHandle` | `{ unsubscribe(): void }` |
+| `MetadataWatchEvent` | `subscribe`'s callback argument: `{ type: 'added' \| 'changed' \| 'deleted', path, name?, stats?, metadataType?, data?, timestamp? }` |
 | `PackagePublishResult` | `{ success, packageId, version, publishedAt, itemsPublished, validationErrors? }` |
 
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