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/core/test/core_test.dart

@@ -15,7 +15,7 @@ void main() {
     test('fromMap tolerates missing keys', () {
       final c = AppConfig.fromMap({});
       expect(c.port, 8080);
-      expect(c.name, 'room');
+      expect(c.name, 'server');
     });
 
     test('round-trips through map', () {
@@ -24,8 +24,10 @@ void main() {
           secret: 's',
           name: 'n',
           mediaPath: 'm',
-          advertiseHost: '1.2.3.4');
+          advertiseHost: '1.2.3.4',
+          hostKey: 'hk');
       final back = AppConfig.fromMap(c.toMap());
+      expect(back.hostKey, 'hk');
       expect(back.port, 1234);
       expect(back.secret, 's');
       expect(back.mediaPath, 'm');
@@ -64,15 +66,29 @@ void main() {
       final back = HelloMessage.fromJson(Map<String, Object?>.from(msg.toJson()));
       expect(back.user.name, 'alice');
       expect(back.media.size, 1500000000);
+      expect(back.hostKey, isEmpty);
+      expect(msg.toJson().containsKey('hostKey'), isFalse);
+    });
+
+    test('hello carries hostKey when set', () {
+      const msg = HelloMessage(
+        proto: 1,
+        secret: '',
+        user: HelloUser(name: 'host'),
+        media: HelloMedia(name: '', size: 0, duration: 0),
+        hostKey: 'hk',
+      );
+      final back = HelloMessage.fromJson(Map<String, Object?>.from(msg.toJson()));
+      expect(back.hostKey, 'hk');
     });
 
     test('hello rejects wrong type', () {
       expect(() => HelloMessage.fromJson({'t': 'bye'}), throwsA(isA<ProtocolException>()));
     });
 
-    test('room state round-trips', () {
-      final s = RoomState(position: 12.5, paused: false, mediaId: 3, serverTime: DateTime.utc(2026, 1, 1));
-      final back = RoomState.fromJson(Map<String, Object?>.from(s.toJson()));
+    test('sync state round-trips', () {
+      final s = SyncState(position: 12.5, paused: false, mediaId: 3, serverTime: DateTime.utc(2026, 1, 1));
+      final back = SyncState.fromJson(Map<String, Object?>.from(s.toJson()));
       expect(back.position, 12.5);
       expect(back.paused, isFalse);
       expect(back.mediaId, 3);

packages/core/test/sync_engine_test.dart

@@ -5,12 +5,12 @@ import 'package:test/test.dart';
 
 void main() {
   late DateTime t;
-  late RoomEngine engine;
+  late SyncEngine engine;
   DateTime clock() => t;
 
   setUp(() {
     t = DateTime.utc(2026, 1, 1);
-    engine = RoomEngine(clock: clock);
+    engine = SyncEngine(clock: clock);
   });
 
   String join(String token, String name) => engine.applyJoin(token, name, '');
@@ -37,12 +37,12 @@ void main() {
   });
 
   group('membership + gate', () {
-    test('join lands out-of-set, room stays held', () {
+    test('join lands out-of-set, playback stays held', () {
       join('a', 'alice');
       expect(engine.transport, Transport.held);
       expect(engine.members['a']!.inSet, isFalse);
-      final room = engine.broadcast();
-      expect(room.members.single.inSet, isFalse);
+      final state = engine.broadcast();
+      expect(state.members.single.inSet, isFalse);
     });
 
     test('single ready + buffered auto-plays', () {
@@ -54,7 +54,7 @@ void main() {
       expect(engine.transport, Transport.playing);
     });
 
-    test('late joiner does not halt a playing room', () {
+    test('late joiner does not halt playback', () {
       join('a', 'alice');
       engine.applyReady('a', true);
       buffer('a', 4.0);
@@ -77,7 +77,7 @@ void main() {
       expect(engine.members['b']!.inSet, isTrue);
     });
 
-    test('disconnect removes from quorum, room resumes', () {
+    test('disconnect removes from quorum, playback resumes', () {
       join('a', 'alice');
       join('b', 'bob');
       engine.applyReady('a', true);
@@ -218,7 +218,7 @@ void main() {
       expect(engine.members['a']!.ready, isTrue);
     });
 
-    test('unpause uses room clock, laggards seek forward', () {
+    test('unpause uses server clock, laggards seek forward', () {
       join('a', 'alice');
       join('b', 'bob');
       engine.applyReady('a', true);
@@ -355,29 +355,29 @@ void main() {
 
   group('tiered correction', () {
     test('big drift seeks either direction', () {
-      expect(correctionFor(ownPosition: 100, roomPosition: 0), Correction.seek);
-      expect(correctionFor(ownPosition: 0, roomPosition: 100), Correction.seek);
+      expect(correctionFor(ownPosition: 100, serverPosition: 0), Correction.seek);
+      expect(correctionFor(ownPosition: 0, serverPosition: 100), Correction.seek);
     });
 
     test('ahead beyond 1.5s slows to 0.95x', () {
-      expect(correctionFor(ownPosition: 3, roomPosition: 0), Correction.slowDown);
-      expect(correctionFor(ownPosition: 1.6, roomPosition: 0), Correction.slowDown);
+      expect(correctionFor(ownPosition: 3, serverPosition: 0), Correction.slowDown);
+      expect(correctionFor(ownPosition: 1.6, serverPosition: 0), Correction.slowDown);
     });
 
     test('deadzone does nothing', () {
-      expect(correctionFor(ownPosition: 1.0, roomPosition: 0), Correction.none);
-      expect(correctionFor(ownPosition: 0, roomPosition: 3.0), Correction.none);
-      expect(correctionFor(ownPosition: 5.1, roomPosition: 0), Correction.seek);
-      expect(correctionFor(ownPosition: 5.0, roomPosition: 0), Correction.slowDown);
+      expect(correctionFor(ownPosition: 1.0, serverPosition: 0), Correction.none);
+      expect(correctionFor(ownPosition: 0, serverPosition: 3.0), Correction.none);
+      expect(correctionFor(ownPosition: 5.1, serverPosition: 0), Correction.seek);
+      expect(correctionFor(ownPosition: 5.0, serverPosition: 0), Correction.slowDown);
     });
   });
 
   group('broadcast shape', () {
-    test('room JSON round-trips with member views', () {
+    test('state JSON round-trips with member views', () {
       join('a', 'alice');
-      final room = engine.broadcast();
+      final state = engine.broadcast();
       final back =
-          RoomBroadcast.fromJson(Map<String, Object?>.from(room.toJson()));
+          StateBroadcast.fromJson(Map<String, Object?>.from(state.toJson()));
       expect(back.transport, 'held');
       expect(back.members.single.name, 'alice');
       expect(back.members.single.id, 'm1');