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/test/session_logic_test.dart

@@ -2,7 +2,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:sync_relay_core/core.dart';
 import 'package:watch_together/src/session/idle.dart';
 import 'package:watch_together/src/session/session_coordinator.dart';
-import 'package:watch_together/src/sync/room_diff.dart';
+import 'package:watch_together/src/sync/state_diff.dart';
 
 MemberView _self({bool ready = true, bool inSet = true, bool stale = false}) =>
     MemberView(