Fan N per-doc stores into one useSyncExternalStore source keyed by id: one
subscribe across all entries, and a memoised Map snapshot rebuilt only when
an entry changes. Each adapter supplies the store-specific per-doc
DocStore (App SDK getDocumentState, Studio editState), while the
shared observer layer owns fan-out and snapshots.
Fan N per-doc stores into one
useSyncExternalStoresource keyed by id: one subscribe across all entries, and a memoisedMapsnapshot rebuilt only when an entry changes. Each adapter supplies the store-specific per-doc DocStore (App SDKgetDocumentState, StudioeditState), while the shared observer layer owns fan-out and snapshots.