Server on an isolate, dedicated server binary, room -> server (proto 5)

- serve(Map config, events) is the one server entrypoint: the app
  hosts it via Isolate.spawn (crash surfaced on the Host card, logs
  forwarded as [server]); bin/server.dart is the dedicated server,
  compiled into the bundle as watch-together-server. --headless gone.
- Host identity via a per-hosting hostKey in hello, replacing the
  loopback heuristic.
- Rename room -> server across code, files and wire (t:"state",
  /health name); protocol bumped to 5.
- CI compiles the server binary; isolate smoke tests in server and
  desktop suites. Docs updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/session/idle.dart

@@ -19,7 +19,7 @@ class IdleTracker {
   }
 
   /// Called once per tick. Warn and unready only matter while self is
-  /// ready; while the room plays the ready flag gates nothing.
+  /// ready; while playing the ready flag gates nothing.
   IdleEvent tick(DateTime now, {required bool playing, required bool selfReady}) {
     if (playing) {
       activity(now);