MetadataManager.register() never notifies subscribe() watchers — runtime consumers go stale on every non-saveMetaItem write

[os-zhuang]

Found while fixing the 15.1 third-party eval P2 "hot-added object never becomes queryable" (framework#3108).

MetadataManager.register() (packages/metadata/src/metadata-manager.ts:280) updates the in-memory registry, persists to writable loaders, and publishes to the realtime service — but it never calls notifyWatchers(), so subscribe(type, cb) consumers are silent. Watchers only fire from the saveMetaItem path (metadata-manager.ts:1590) and NodeMetadataManager's file watcher.

Concretely: ObjectQL's registry bridge (subscribeToMetadataEvents, packages/objectql/src/plugin.ts) — the component that keeps the SchemaRegistry in sync — never hears about anything registered through register(), which is exactly what the artifact reload path (_parseAndRegisterArtifact) uses. #3108 works around this for the metadata:reloaded seam by carrying the parsed artifact on the event payload and ingesting from there, but the asymmetry remains for any other register() call site (e.g. a future cloud artifact-api mid-run refresh) and for any other subscribe() consumer.

Options:

1. Make register() emit the same MetadataWatchEvent as saveMetaItem (blast radius: boot-time registration would flood subscribers with N events — needs either a bulk/quiet mode for boot or subscribers that tolerate the flood).
2. Keep register() silent by design, document it loudly on the method, and require every bulk-ingest path to announce (as metadata:reloaded now does).

Either way the contract should be explicit — today it's a trap: subscribe() looks like it covers all writes, and it doesn't.

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