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.md

@@ -1,6 +1,6 @@
 ---
 status: draft
-last_updated: 2026-05-07
+last_updated: 2026-05-08
 ---
 
 # Event Target Adapters
@@ -21,11 +21,41 @@ Every adapter must implement:
 
 Adapters come in two shapes:
 
-- **Symmetric** (single connection) — wraps one connection. Same interface on both sides. Examples: Redis, Iroh spoke, WebSocket client, Worker main-thread.
-- **Fan-out** (multi-connection) — manages multiple connections. `dispatchEvent` sends to all; `addEventListener` aggregates from all. Examples: WebSocket server, Iroh hub, Worker pool manager.
+- **Symmetric** (single connection) — wraps one connection. Same interface on both sides. Examples: Redis, Iroh spoke, WebSocket client, Worker.
+- **Fan-out** (multi-connection) — manages multiple connections. `dispatchEvent` sends only to connections subscribed to the event type (topic-based fan-out); `addEventListener` aggregates from all. Examples: WebSocket server, Iroh hub.
+
+Worker is a symmetric adapter (point-to-point). A Worker pool manager would be a fan-out concern, but that belongs in `@alkdev/operations`, not in this package.
 
 The `createPubSub` layer is topology-agnostic. A hub composes multiple adapters and uses operators to combine streams — this is downstream application logic, not a package boundary.
 
+## Composing Adapters
+
+Adapters compose at the `createPubSub` level, not by nesting adapters. A common pattern:
+
+```
+Client (WS spoke) → Server (WS hub) → Redis → Other servers
+```
+
+In this topology:
+- The WS hub adapter manages spoke connections with topic-based fan-out
+- The hub's `createPubSub` instance also has a Redis event target for cross-server communication
+- Subscription state at the hub level aggregates: when a spoke subscribes to topic `X`, the hub may also subscribe to `X` on Redis (if no other spoke is already subscribed)
+
+This composition is handled by `createPubSub` bridging multiple event targets, not by adapters wrapping each other. The fan-out adapter's subscription tracking is local to its connected spokes; upstream subscriptions are a `createPubSub` concern.
+
+## Subscription Control Protocol
+
+Fan-out adapters must know which connections are interested in which topics. Symmetric adapters that connect to a fan-out adapter must declare their subscriptions. This is done through **control events** — `EventEnvelope` messages with reserved `__`-prefixed types:
+
+- `__subscribe` — sent when a spoke adds the first listener for a topic
+- `__unsubscribe` — sent when a spoke removes the last listener for a topic
+
+The `id` field in control events is the empty string (`""`) by convention. Control events use the `topic` field in their payload for routing instead. Control events are handled internally by adapters and never dispatched to user-facing listeners. Event types starting with `__` are reserved — user code must not define event types with this prefix.
+
+See [ADR-003](decisions/003-subscription-control-protocol.md).
+
+This is analogous to Redis's `SUBSCRIBE`/`UNSUBSCRIBE` commands — control messages share the same wire format and connection as data.
+
 ## Adapter Docs
 
 | Adapter | Import | Status |