Add per-adapter architecture docs in event-targets/ directory

- Create docs/architecture/event-targets/ with individual specs:
  in-process, redis, websocket-client, websocket-server,
  worker, iroh-spoke, iroh-hub
- Update event-targets.md to serve as index with topology model
  (symmetric vs fan-out) and adapter status table
- Update architecture.md index to reference new directory

docs/architecture/event-targets/in-process.md

@@ -0,0 +1,25 @@
+---
+status: draft
+last_updated: 2026-05-07
+---
+
+# In-Process Event Target
+
+No adapter needed. `createPubSub` uses `new EventTarget()` by default.
+
+This works for single-process deployments where all pubsub participants share the same memory. The web standard `EventTarget` already implements `addEventListener`/`dispatchEvent`/`removeEventListener`.
+
+```ts
+const pubsub = createPubSub<MyEventMap>();
+// uses new EventTarget() by default
+```
+
+Could be formalized as an explicit `InProcessEventTarget` for documentation purposes, but there's no functional need — the browser/Node `EventTarget` is the reference implementation of the `TypedEventTarget` contract.
+
+## Design Notes
+
+- Topics use the `type:id` string convention (e.g., `"call.responded:uuid-123"`)
+- `CustomEvent.detail` carries the full `EventEnvelope` object
+- No serialization — objects are passed by reference
+- Synchronous dispatch — listeners fire immediately in the current call stack
+- No connection lifecycle, no reconnection concerns