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/lib/src/config.dart

@@ -15,6 +15,9 @@ class AppConfig {
   /// Optional explicit host for file-media URLs; empty means use the host the
   /// client dialled.
   final String advertiseHost;
+  /// Secret the hosting app's own client sends in hello to be marked host
+  /// (unkickable). Empty means no member is the host (dedicated server).
+  final String hostKey;
 
   const AppConfig({
     required this.port,
@@ -23,20 +26,22 @@ class AppConfig {
     this.mediaPath = '',
     this.logLevel = 'INFO',
     this.advertiseHost = '',
+    this.hostKey = '',
   });
 
   factory AppConfig.defaults() =>
-      const AppConfig(port: 8080, secret: '', name: 'room');
+      const AppConfig(port: 8080, secret: '', name: 'server');
 
   factory AppConfig.fromMap(Map<String, Object?> map) {
     try {
       return AppConfig(
         port: (map['port'] as num?)?.toInt() ?? 8080,
         secret: (map['secret'] as String?) ?? '',
-        name: (map['name'] as String?) ?? 'room',
+        name: (map['name'] as String?) ?? 'server',
         mediaPath: (map['mediaPath'] as String?) ?? '',
         logLevel: (map['logLevel'] as String?) ?? 'INFO',
         advertiseHost: (map['advertiseHost'] as String?) ?? '',
+        hostKey: (map['hostKey'] as String?) ?? '',
       );
     } catch (e) {
       throw ConfigException('invalid config map: $e');
@@ -50,6 +55,7 @@ class AppConfig {
         'mediaPath': mediaPath,
         'logLevel': logLevel,
         'advertiseHost': advertiseHost,
+        'hostKey': hostKey,
       };
 
   static Future<AppConfig> loadFile(String path) async {

packages/core/lib/src/constants.dart

@@ -1,5 +1,5 @@
 abstract class RelayConstants {
-  static const int protoVersion = 4;
+  static const int protoVersion = 5;
 
   static const int chunkBytes = 256 * 1024;
   static const int creditWindowChunks = 8;
@@ -19,7 +19,7 @@ abstract class RelayConstants {
   // leaves wide headroom over any 128x128 pipeline PNG (raw bitmap
   // <= 64 KiB, worst-case base64 ~95 KiB chars) — deliberate over-size
   // for private-server use. PNG keeps transparency; JPEG did not.
-  // Trade: avatars ride in the 1 Hz room broadcast, so worst-case host
+  // Trade: avatars ride in the 1 Hz state broadcast, so worst-case host
   // uplink scales with this cap. Follow-up (not now): send avatar bytes
   // once, reference by hash afterwards.
   static const int maxAvatarBytes = 256 * 1024;

packages/core/lib/src/protocol.dart

@@ -67,7 +67,15 @@ class HelloMessage {
   final String secret;
   final HelloUser user;
   final HelloMedia media;
-  const HelloMessage({required this.proto, required this.secret, required this.user, required this.media});
+  /// The hosting app's own client proves it is the host with this; empty
+  /// for everyone else.
+  final String hostKey;
+  const HelloMessage(
+      {required this.proto,
+      required this.secret,
+      required this.user,
+      required this.media,
+      this.hostKey = ''});
 
   Map<String, Object?> toJson() => {
         't': 'hello',
@@ -75,6 +83,7 @@ class HelloMessage {
         'secret': secret,
         'user': user.toJson(),
         'media': media.toJson(),
+        if (hostKey.isNotEmpty) 'hostKey': hostKey,
       };
 
   factory HelloMessage.fromJson(Map<String, Object?> json) {
@@ -87,16 +96,17 @@ class HelloMessage {
       secret: (json['secret'] as String?) ?? '',
       user: HelloUser.fromJson(Map<String, Object?>.from(user)),
       media: HelloMedia.fromJson(Map<String, Object?>.from(media)),
+      hostKey: (json['hostKey'] as String?) ?? '',
     );
   }
 }
 
-class RoomState {
+class SyncState {
   final double position;
   final bool paused;
   final int mediaId;
   final DateTime serverTime;
-  const RoomState({required this.position, required this.paused, required this.mediaId, required this.serverTime});
+  const SyncState({required this.position, required this.paused, required this.mediaId, required this.serverTime});
 
   Map<String, Object?> toJson() => {
         'position': position,
@@ -105,7 +115,7 @@ class RoomState {
         'serverTime': serverTime.toIso8601String(),
       };
 
-  factory RoomState.fromJson(Map<String, Object?> json) => RoomState(
+  factory SyncState.fromJson(Map<String, Object?> json) => SyncState(
         position: finiteDouble(json['position'], 'position'),
         paused: (json['paused'] as bool?) ?? true,
         mediaId: finiteInt(json['mediaId'], 'mediaId', fallback: 0),
@@ -296,7 +306,7 @@ class MemberView {
       );
 }
 
-class RoomBroadcast {
+class StateBroadcast {
   final int eventId;
   final String transport;
   final double position;
@@ -304,7 +314,7 @@ class RoomBroadcast {
   final int mediaId;
   final Map<String, Object?>? media;
   final List<MemberView> members;
-  const RoomBroadcast({
+  const StateBroadcast({
     required this.eventId,
     required this.transport,
     required this.position,
@@ -315,7 +325,7 @@ class RoomBroadcast {
   });
 
   Map<String, Object?> toJson() => {
-        't': 'room',
+        't': 'state',
         'eventId': eventId,
         'transport': transport,
         'position': position,
@@ -325,11 +335,11 @@ class RoomBroadcast {
         'members': members.map((m) => m.toJson()).toList(),
       };
 
-  factory RoomBroadcast.fromJson(Map<String, Object?> json) {
-    if (json['t'] != 'room') throw const ProtocolException('not a room broadcast');
+  factory StateBroadcast.fromJson(Map<String, Object?> json) {
+    if (json['t'] != 'state') throw const ProtocolException('not a state broadcast');
     final members = json['members'];
     final media = json['media'];
-    return RoomBroadcast(
+    return StateBroadcast(
       eventId: finiteInt(json['eventId'], 'eventId', fallback: 0),
       transport: (json['transport'] as String?) ?? 'held',
       position: finiteDouble(json['position'], 'position'),

packages/core/lib/src/sync_engine.dart

@@ -1,14 +1,14 @@
 import 'constants.dart';
 import 'protocol.dart';
 
-typedef RoomClock = DateTime Function();
+typedef SyncClock = DateTime Function();
 
 enum Transport { playing, held }
 
 enum Correction { none, slowDown, seek }
 
-Correction correctionFor({required double ownPosition, required double roomPosition}) {
-  final drift = ownPosition - roomPosition;
+Correction correctionFor({required double ownPosition, required double serverPosition}) {
+  final drift = ownPosition - serverPosition;
   if (drift.abs() > RelayConstants.driftSeekThresholdSecs) return Correction.seek;
   if (drift > RelayConstants.slowDownThresholdSecs) return Correction.slowDown;
   return Correction.none;
@@ -52,8 +52,8 @@ class EngineFeed {
   }
 }
 
-class RoomEngine {
-  final RoomClock clock;
+class SyncEngine {
+  final SyncClock clock;
   final Duration staleAfter;
   final Duration dropAfter;
   final EngineFeed feed = EngineFeed();
@@ -67,7 +67,7 @@ class RoomEngine {
   HelloMedia? media;
   int _nextId = 1;
 
-  RoomEngine({RoomClock? clock, Duration? staleAfter, Duration? dropAfter})
+  SyncEngine({SyncClock? clock, Duration? staleAfter, Duration? dropAfter})
       : clock = clock ?? (() => DateTime.now().toUtc()),
         staleAfter = staleAfter ?? const Duration(seconds: 15),
         dropAfter = dropAfter ?? const Duration(seconds: 30),
@@ -249,7 +249,7 @@ class RoomEngine {
     }
   }
 
-  RoomBroadcast broadcast() => RoomBroadcast(
+  StateBroadcast broadcast() => StateBroadcast(
         eventId: eventId,
         transport: transport == Transport.playing ? 'playing' : 'held',
         position: position,