Resolve WebSocket event target open questions, add subscription control protocol

- Resolve OQ1: WS server accepts raw WebSocket instances via
  addConnection/removeConnection (framework-agnostic, not coupled to
  Hono/Express/Bun/Deno)
- Resolve OQ2: Backpressure handled by disconnecting slow consumers at
  configurable threshold (default 1MB), with onBackpressure callback
  for observability
- Resolve OQ3: Topic-based fan-out with subscription tracking instead
  of broadcast-all; spokes send __subscribe/__unsubscribe control
  events; direct messaging via 'direct:' topic pattern

Add ADR-003 for subscription control protocol decision. Update all
fan-out adapters (WS server, Iroh hub) and spoke adapters (WS client,
Iroh spoke) with subscription tracking/forwarding. Fix routing key
ambiguity (full topic string, not event type alone). Add error
handling, composition, and reserved type sections. Clarify Worker as
symmetric-only.

docs/architecture/event-targets/iroh-spoke.md

@@ -1,6 +1,6 @@
 ---
 status: draft
-last_updated: 2026-05-07
+last_updated: 2026-05-08
 ---
 
 # Iroh Spoke Event Target
@@ -49,12 +49,24 @@ The JSON payload is the `EventEnvelope`:
 4. Spoke wraps streams in `IrohSpokeEventTarget`
 5. On disconnect: `RecvStream.readExact()` throws, spoke must reconnect
 
+## Subscription Forwarding
+
+Same pattern as [WebSocket Client](websocket-client.md): when `addEventListener` is called for the first listener on a topic, the spoke sends a `__subscribe` control event to the hub. When `removeEventListener` removes the last listener for a topic, the spoke sends `__unsubscribe`. Reference counting ensures `__subscribe` is sent only on the first `addEventListener` for a topic, and `__unsubscribe` only when the last `removeEventListener` for that topic is called. This enables the hub's topic-based fan-out (see [ADR-003](../decisions/003-subscription-control-protocol.md)).
+
+Control events use the existing `EventEnvelope` format:
+
+```json
+{ "type": "__subscribe", "id": "", "payload": { "topic": "message.sent:conv-123" } }
+{ "type": "__unsubscribe", "id": "", "payload": { "topic": "message.sent:conv-123" } }
+```
+
 ## Key Properties
 
 - **NAT traversal** — spoke dials hub by `NodeId`, no public IP needed
 - **Cryptographic identity** — `Connection.remoteNodeId()` verifies the hub
 - **Bidirectional** — `dispatchEvent` writes to `SendStream`, `addEventListener` reads from `RecvStream`
 - **Per-connection** — one event target per QUIC connection
+- **Subscription forwarding** — `addEventListener`/`removeEventListener` send `__subscribe`/`__unsubscribe` control events to the hub (see [ADR-003](../decisions/003-subscription-control-protocol.md))
 
 ## R&D Needed