docs(roadmap): ratify v0.6 Channels design

User-ratified 2026-06-12. Resolves the v0.6 fork left by the v0.5
handoff: dep #4 is serde/serde_json, but ONLY behind an opt-in
'phoenix' feature (V2 codec + JsonCodec). The 'channels' core takes
zero new deps — payload generic over urus Encode/Decode traits.
Channel trait shape, ChannelSocket, TopicRouter/PrefixRouter, and
opt-in ChannelSession persistence (zero-copy Arc<M> buffer drain)
all specified in the v0.6 section.

ROADMAP.md

@@ -265,18 +265,69 @@ pid) + spawns the relay, `broadcast_from(self_pid(), ..)` for no-echo.
 
 ## v0.6 — Channels
 
-Phoenix channels: the join/leave/event protocol over WebSocket transport,
-PubSub underneath.
+Join/leave/event protocol over WebSocket transport, PubSub underneath.
+Design decisions ratified pre-code (2026-06-12):
 
-- `Channel` trait: `join(topic, payload, socket)`, `handle_in(event,
-  payload, socket)`, `handle_out`, `terminate`.
-- Topic router (`"room:*"` patterns) → channel actor per (conn, topic),
-  spawned under the ws connection, linked so conn death reaps channels.
-- Wire format: Phoenix V2 JSON serializer
-  (`[join_ref, ref, topic, event, payload]`) — free interop with
-  phoenix.js clients is the killer feature; needs a JSON dep or a
-  hand-rolled mini-codec (decision point — this would be dep #3).
-- Presence: explicitly out of scope until distribution exists somewhere.
+**Feature flags.** Two additive features:
+- `"channels"` — the `Channel` trait, `TopicRouter` trait, `PrefixRouter`,
+  `ChannelSocket`, session machinery. Zero new deps; pubsub is already
+  always-on (it only exists because channels needs it).
+- `"phoenix"` — implies `"channels"` + `serde`/`serde_json` + the V2 frame
+  codec. Opt-in phoenix.js interop; users who want protobuf or any other
+  wire format take `"channels"` only and supply their own codec.
+
+**Payload generics.** The `Channel` trait is generic over payload:
+`P: Encode + Decode` where `Encode`/`Decode` are urus codec traits (one
+method each). The `"phoenix"` feature provides a `JsonCodec` blanket impl
+via serde. No JSON anywhere in the `"channels"` surface.
+
+**`Channel` trait shape.**
+```
+trait Channel<P>: Send + 'static {
+    fn join(topic: &str, payload: P, socket: &ChannelSocket<P>) -> Result<P, P>;
+    fn handle_in(&mut self, event: &str, payload: P, socket: &ChannelSocket<P>);
+    fn terminate(&mut self) {}
+}
+```
+`handle_out` deferred — can land as a defaulted method, non-breaking.
+
+**`ChannelSocket`.** Newtype over `WsSender` + a `PubSub` handle.
+Exposes `reply(ref, status, payload)`, `push(event, payload)`,
+`broadcast(topic, payload)`. The `ref` from the incoming frame is threaded
+in by the channel actor loop, invisible to the impl. Coupling to `PubSub`
+is intentional: channels owns pubsub, and broadcast is the core use case.
+
+**Topic router.** `TopicRouter` trait: `fn route(&self, topic: &str) ->
+Option<Box<dyn ChannelFactory<P>>>`. Factory receives the full raw topic
+string so wildcard-segment extraction is always possible. urus ships
+`PrefixRouter` as the default impl: scan to `'*'`, discard the rest,
+single `HashMap` lookup on the prefix. No regex dep, no glob machinery —
+users who need that implement `TopicRouter` themselves.
+
+**Channel actor lifetime + session persistence.**
+Default: channel actor linked to the ws connection actor, cold-start on
+every reconnect (phoenix-server-compatible behavior). Opt-in persistence
+via a `ChannelSession` trait:
+```
+trait ChannelSession: Send + 'static {
+    type Key: Eq + Hash + Send + 'static;
+    fn session_key(topic: &str, join_payload: &P) -> Self::Key;
+    fn buffer_cap() -> usize { 128 }
+    fn ttl() -> Duration { Duration::from_secs(30) }
+}
+```
+When implemented, the router consults a session registry (gen_server,
+same pattern as the connection registry) before spawning: an existing
+actor is reattached, and buffered outbound messages (`Vec<Arc<M>>`, no
+re-serialisation) are drained to the reconnecting transport. Buffer full
+or TTL expired → actor tears down; next join is a cold start. This
+diverges from Phoenix server conventions (client-re-syncs) but is
+invisible to phoenix.js at the wire level. Zero-copy drain is the smarm
+motivation: messages are `Arc<M>` in the buffer and in the PubSub relay
+path, one allocation per broadcast regardless of subscriber count or
+reconnect cycles.
+
+**Presence:** explicitly out of scope until distribution exists.
 
 ---