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/main.dart

@@ -19,10 +19,6 @@ Future<void> main(List<String> args) async {
     exit(exitCode);
   }
   initLogging();
-  if (args.contains('--headless')) {
-    await runHeadless(args);
-    return;
-  }
   FlutterError.onError = (details) {
     debugPrint('FLUTTER ERROR: ${details.exceptionAsString()}\n${details.stack}');
     FlutterError.presentError(details);
@@ -67,13 +63,6 @@ Future<void> main(List<String> args) async {
   runApp(const WatchTogetherApp());
 }
 
-Future<void> runHeadless(List<String> args) async {
-  final session = SessionState.instance;
-  await session.startHeadless(args);
-  await ProcessSignal.sigint.watch().first;
-  await session.stopServer();
-}
-
 /// Close button / Alt+F4: leave gracefully first (others see "left", a
 /// hosted server shuts down), bounded so a hung socket can't keep the
 /// window open.