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/session_test.dart

@@ -80,8 +80,8 @@ void main() {
   });
 
   group('core welcome + proto', () {
-    test('proto bumped to 4 for readiness + room fan-out', () {
-      expect(RelayConstants.protoVersion, 4);
+    test('proto 5: state broadcast + hello hostKey', () {
+      expect(RelayConstants.protoVersion, 5);
     });
 
     test('welcome round-trips with token + member id', () {