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/lifecycle_test.dart

@@ -60,10 +60,10 @@ Future<void> _pollBool(
 
 void main() {
   group('presence lifecycle live', () {
-    late RoomServer server;
+    late SyncServer server;
 
     setUp(() async {
-      server = RoomServer(
+      server = SyncServer(
           const AppConfig(port: 0, secret: 'cycle', name: 'cycle'));
       await server.start();
     });