[P1-5] Trajectory subscription stream: tokio broadcast + SDK trajectory.subscribe

[justrach]

Parent: #34
Priority: P1 (live data flow — graff-memd needs this to react in real time)
Branch base: release/0.1.53

Why

Today TrajectoryRecorder writes to SQLite. graff-memd needs to subscribe to those events live without polling. Hermes does this in-process; we need a broadcast channel that crosses the N-API boundary.

Scope

Rust side

• TrajectoryRecorder gets a tokio::sync::broadcast::Sender<TrajectoryEvent> alongside its repo write path. Every recorded event is also broadcast.
• New API on forge_api::ForgeAPI: subscribe_trajectory(conversation_id: Option<ConversationId>) -> Receiver<TrajectoryEvent>.

SDK side (sdk/typescript/src/lib.rs + wire.rs)

• #[napi] method on GraffApi: subscribeTrajectory(conversationId?: string): TrajectoryStream.
• TrajectoryStream exposes .next() returning a JSON-serialized event (mirror ChatStreamHandle).
• Public TS:

  graff.trajectory.subscribe(conversationId?: string): AsyncIterable<TrajectoryEvent>

Acceptance

• Two subscribers see the same events
• Subscriber that lags doesn't block the recorder (use RecvError::Lagged handling — emit a lag event and continue)
• for await ... break cancels cleanly (no hanging tokio task)
• Smoke test in examples/trajectory-subscribe.ts