shell.annotations.onChanged: Event Bus consumer in os-sdk

[timkicker]

Goal

Implement the shell.annotations.onChanged({ target, namespace, handle }) API per foundation §419. Apps subscribe to annotation changes by target+namespace and receive callbacks when matching events flow through the Event Bus.

Background

Sprint that closed sdk#15 landed set / get / clear for shell.annotations, but onChanged was deferred because os-sdk has only a producer-side Event Bus interface (UnixEventEmitter). Subscribing requires a consumer-side counterpart.

Scope

1. os-sdk Event Bus consumer — new module sdk/os-sdk/src/event_consumer.rs:

   • EventConsumer trait + UnixEventConsumer impl
   • Connects to the consumer socket (/run/lunaris/event-bus-consumer.sock), registers with a stable consumer id derived from app_id, subscribes to event-type prefixes
   • Returns an async-stream-ish or callback-based interface
   • Mirror MockEventEmitter with MockEventConsumer for tests

2. Annotations::on_changed in os-sdk:

   • Spawns a consumer subscription filtered to app.annotation.set and app.annotation.cleared
   • Filters by target + namespace client-side (cheap; the daemon broadcasts all matching apps' events)
   • Returns a handle that drops the subscription when dropped (RAII)

3. tauri-plugin-shell: shell_annotation_subscribe Tauri command that wires onChanged callback through Tauri events (lunaris://annotation-changed)

4. TS surface: shell.annotations.onChanged({ target, namespace }, handle) returns an unsubscribe function

Out of scope

• Cross-namespace subscription rate-limiting (defer to capability-token Phase 3.2-full)
• onChanged for non-annotation event types (presence/timeline) — those have no defined onChanged in foundation

Acceptance

• App calls shell.annotations.onChanged; another app calls shell.annotations.set with matching target+namespace; first app's handler fires within ~100ms
• Drop / unsubscribe correctly tears down the subscription (no event leak in tests)
• Tests cover: matching event fires, non-matching event ignored, drop unsubscribes

Phase

Phase 6 follow-up. Discovered during sdk#15 sprint as deferred scope.