Event/hook system for record changes

[cuibonobo]

There is currently no way to react to record lifecycle events. Apps can't observe when records are created, updated, or deleted without polling.

This matters for several use cases:

• Full-text or secondary indexing — rebuild an index when a record changes
• Sync — propagate local writes to a remote when the adapter-api lands
• Audit logging — record who changed what and when
• Reactive UI — invalidate caches or trigger re-renders on write

Proposed API (sketch):

stack.on('record:create', (record) => { ... });
stack.on('record:update', (record, previous) => { ... });
stack.on('record:delete', (id, { hard }) => { ... });
// or filtered by type:
stack.on('record:update', { typeId: 'com.example.app/note@1' }, (record) => { ... });

Hooks could be synchronous (fire-and-forget) or async (awaited before the write completes — useful for indexing). The right model needs thought, especially around error handling if a hook throws.

The adapter interface may also need a hook mechanism so that adapters (particularly adapter-api) can emit events from remote changes, not just local writes.