Restructure step 3: your server, Invite, version gate, AppGraph

- One server per app run (loopback, no password); Invite attaches an
  all-interfaces listener to the same server and Stop sharing detaches
  it (guests told with 4016); members, media and Up next never reset.
  No solo mode: you're home on your own server unless in a friend's.
- Navigation: none / yours / friend; a join passes a version gate on
  /health first (clear "they/you need to update", update offer).
- Invite as a value (host, port, name, password flag, version, fp
  reserved); addresses filtered (loopback, link-local, 198.18/15,
  bridges) and ranked; Sharing chip, Copy invite, Stop sharing.
- Alone looks like a plain player (alone replaces solo).
- AppGraph builds and disposes systems; widgets use AppScope;
  SessionState deleted; a test caps the remaining singletons.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/core/lib/src/protocol.dart

@@ -667,6 +667,9 @@ abstract class CloseCodes {
   static const stale = 4014;
   /// `/sync`: no frame for the silence limit; the member is `gone`.
   static const silent = 4015;
+  /// `/sync`: the hosting app stopped sharing its server; the member has
+  /// left (feed `leave`, reason `closed`) and nothing reconnects.
+  static const closed = 4016;
   /// `/sync`: the member left (`bye`, or grace over).
   static const left = 1000;
 }