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>

packages/server/test/server_test.dart

@@ -23,7 +23,7 @@ void main() {
   });
 
   test('server starts, answers /health, stops cleanly', () async {
-    final server = RoomServer(const AppConfig(port: 0, secret: 't', name: 'test'));
+    final server = SyncServer(const AppConfig(port: 0, secret: 't', name: 'test'));
     await server.start();
     expect(server.isRunning, isTrue);
     final port = server.boundPort;
@@ -45,7 +45,7 @@ void main() {
   });
 
   test('unknown path returns 404', () async {
-    final server = RoomServer(const AppConfig(port: 0, secret: 't', name: 'test'));
+    final server = SyncServer(const AppConfig(port: 0, secret: 't', name: 'test'));
     await server.start();
     final client = HttpClient();
     try {