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/lib/src/serve.dart

@@ -0,0 +1,47 @@
+import 'dart:async';
+import 'dart:isolate';
+
+import 'package:logging/logging.dart' as pkg_logging;
+import 'package:sync_relay_core/core.dart';
+
+import 'sync_server.dart';
+
+/// The one server entrypoint: the dedicated binary and the app's hosting
+/// isolate both run this. [config] holds primitives only (an
+/// [AppConfig.toMap]) so it can cross an isolate boundary.
+///
+/// On [events] it reports `{'t':'ready','port':p,'stop':SendPort}` or
+/// `{'t':'error','message':m}`, and forwards log records as
+/// `{'t':'log','level','logger','message'[,'error']}`. Sending
+/// `{'t':'stop'}` to the `stop` port stops the server; the returned future
+/// completes once it is down. Without [events] a start failure throws.
+Future<void> serve(Map<String, Object?> config, {SendPort? events}) async {
+  StreamSubscription<pkg_logging.LogRecord>? logs;
+  if (events != null) {
+    logs = pkg_logging.Logger.root.onRecord.listen((r) => events.send({
+          't': 'log',
+          'level': r.level.name,
+          'logger': r.loggerName,
+          'message': r.message,
+          if (r.error != null) 'error': '${r.error}',
+        }));
+  }
+  final SyncServer server;
+  try {
+    server = SyncServer(AppConfig.fromMap(config));
+    await server.start();
+  } catch (e) {
+    await logs?.cancel();
+    if (events == null) rethrow;
+    events.send({'t': 'error', 'message': '$e'});
+    return;
+  }
+  final control = ReceivePort('serve control');
+  events?.send({'t': 'ready', 'port': server.boundPort, 'stop': control.sendPort});
+  await for (final msg in control) {
+    if (msg is Map && msg['t'] == 'stop') break;
+  }
+  control.close();
+  await server.stop();
+  await logs?.cancel();
+}