docs(channels): example, README section, ROADMAP v0.6 mark-done — chunk 4

examples/channels_chat.rs (required-features phoenix): ephemeral
room:* and persistent session:* side by side over phoenix.js V2 JSON,
stdin-driven graceful shutdown (the ws_chat pattern). Smoke-tested
live: rejoin after transport drop lands on the same instance (state
counter persists across three transports), broadcasts carry the new
join generation's ref.

README: channels section (feature split, core trait walkthrough,
session persistence semantics incl. the re-entrant-join contract).
ROADMAP: v0.6 marked done with the full as-landed decision list,
chunked as committed.

ROADMAP.md

@@ -263,7 +263,7 @@ smarm has none). All six design questions ratified by the user pre-code
 Chunk 2's ws_chat: rooms as topics, `on_open` subscribes (conn-actor
 pid) + spawns the relay, `broadcast_from(self_pid(), ..)` for no-echo.
 
-## v0.6 — Channels
+## v0.6 — Channels — DONE (2026-06-12)
 
 Join/leave/event protocol over WebSocket transport, PubSub underneath.
 Design decisions ratified pre-code (2026-06-12):
@@ -329,6 +329,24 @@ reconnect cycles.
 
 **Presence:** explicitly out of scope until distribution exists.
 
+**As-landed (2026-06-12), beyond the ratified text — veto by diff:**
+- chunk 1: `Channel::join` takes `&mut self` (state from join, and
+  session rejoins need the same instance); the in-flight ref is
+  threaded invisibly through the socket; `broadcast` gained an `event`
+  parameter; `TopicRouter::route` returns `Option<Arc<dyn
+  ChannelFactory>>`.
+- chunk 2: the serde blanket lives behind the `Json<T>` newtype
+  (a blanket on bare `P: Serialize` is coherence poison under additive
+  features); encode failure panics into the linked-teardown contract.
+- chunk 3: `ChannelFactory` gained a `#[doc(hidden)] deploy()` seam
+  (default = the ephemeral linked actor); every attach re-calls
+  `join()` (re-entrant by contract); rejected rejoin / explicit leave /
+  cap / TTL all end the session; second transport evicts the first;
+  `Key: Clone` (registry's pid→key reverse index); registry spawns
+  eagerly at registration (in-runtime law, same as `ChannelHub::new`);
+  shutdown composes linklessly via the registry-drop chain — proven by
+  `shutdown_with_detached_session_terminates`.
+
 ---
 
 ## Known bugs