feat(notifications): node-local notif.written hook

NotificationService emits a node-local notif.written event after it writes a notification, so an extension can act on new notifications without core knowing what it does. Part of PUT-2167. Stacks on #4317.

What

• notif.written fires once per recipient for every non-silent write (notify) and for rewrites in place (notifyUpdate, with updated: true). Payload: { userId, uid, type, appUid?, value, updated? }.
• Typed in EventMap and listed under the unpublished internal events in the events registry, so apps can't subscribe to it.
• Silent writes don't emit.

Review follow-ups

• notif.written hands each listener its own copy of the payload, so a listener changing it can't change what the desktop receives.
• The payload now carries audience, and the update emit carries appUid from the row.

Tests

• NotificationService.test.ts gains three cases: one event per recipient with the payload, none for a silent write, and updated: true on a rewrite.
• Notification service, driver and events registry tests: 147 passed. Typecheck: no new errors.