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/serve_isolate_test.dart

@@ -0,0 +1,70 @@
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+import 'dart:isolate';
+
+import 'package:sync_relay_core/core.dart';
+import 'package:sync_relay_server/server.dart';
+import 'package:test/test.dart';
+
+Future<void> _run((Map<String, Object?>, SendPort) args) =>
+    serve(args.$1, events: args.$2);
+
+void main() {
+  test('serve in a real isolate: ready, /health, stop, clean exit', () async {
+    final events = ReceivePort();
+    final exit = ReceivePort();
+    final errors = ReceivePort();
+    final config = const AppConfig(port: 0, secret: '', name: 'iso', hostKey: 'hk').toMap();
+    await Isolate.spawn(_run, (config, events.sendPort),
+        onExit: exit.sendPort, onError: errors.sendPort);
+    final crashes = <Object?>[];
+    errors.listen(crashes.add);
+    final logs = <Map>[];
+    final ready = Completer<Map>();
+    events.listen((m) {
+      if (m is! Map) return;
+      if (m['t'] == 'log') logs.add(m);
+      if (m['t'] == 'ready' && !ready.isCompleted) ready.complete(m);
+      if (m['t'] == 'error' && !ready.isCompleted) ready.completeError('${m['message']}');
+    });
+    final r = await ready.future.timeout(const Duration(seconds: 10));
+    final port = r['port'] as int;
+    expect(port, greaterThan(0));
+
+    final http = HttpClient();
+    final resp = await (await http.get('127.0.0.1', port, '/health')).close();
+    final body = jsonDecode(await resp.transform(utf8.decoder).join()) as Map;
+    http.close();
+    expect(body['ok'], isTrue);
+    expect(body['name'], 'iso');
+    expect(body['proto'], RelayConstants.protoVersion);
+    expect(logs.any((l) => '${l['message']}'.contains('listening on $port')), isTrue);
+
+    (r['stop'] as SendPort).send({'t': 'stop'});
+    await exit.first.timeout(const Duration(seconds: 10));
+    expect(crashes, isEmpty);
+    // The port is free again.
+    final again = await ServerSocket.bind(InternetAddress.anyIPv4, port);
+    await again.close();
+    events.close();
+    errors.close();
+  });
+
+  test('serve reports a bind failure as an error event', () async {
+    final blocker = await ServerSocket.bind(InternetAddress.anyIPv4, 0);
+    final events = ReceivePort();
+    final exit = ReceivePort();
+    await Isolate.spawn(
+        _run,
+        (AppConfig(port: blocker.port, secret: '', name: 'x').toMap(),
+            events.sendPort),
+        onExit: exit.sendPort);
+    final first = await events
+        .firstWhere((m) => m is Map && m['t'] != 'log')
+        .timeout(const Duration(seconds: 10)) as Map;
+    expect(first['t'], 'error');
+    await exit.first.timeout(const Duration(seconds: 10));
+    await blocker.close();
+  });
+}