Plugin seam for #621 (LWS), under #206.

Problem

src/notifications/events.js already emits resource change events for the solid-0.1 WebSocket channel, but the emitter isn't reachable from a plugin. LWS webhook notifications (and ActivityPub outbox fan-out, search indexing, type index #535, …) all need "tell me when a resource changes".

Proposal

const off = api.events.on('change', ({ type, url, isContainer, agent, at }) => { ... });
// type: 'create' | 'update' | 'delete'

• Fires after the write has committed, off the request path (setImmediate / queueMicrotask), so a slow listener can't delay the response.
• Listener exceptions are caught and logged with the plugin id.
• off() unsubscribes; deactivate() should call it, but the loader also drops all listeners registered by a plugin on close.
• Mirrors what the WebSocket notifier consumes today — no new event source, just exposure.

Prior art: LwsNotificationListener in jeswr/LinkedWebStorage, which turns changes into Create/Update/Delete activities, matches recursive container topics, and re-checks read permission at delivery time.