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>

apps/desktop/test/volume_readout_test.dart

@@ -6,16 +6,20 @@ import 'package:media_kit_video/media_kit_video.dart' show VideoState;
 import 'package:starsync/src/controls/model/controls_prefs.dart';
 import 'package:starsync/src/controls/frame.dart';
 import 'package:starsync/src/shell/session_player.dart';
-import 'package:starsync/src/state/session_state.dart';
 import 'package:starsync/src/state/settings.dart';
 
+import 'test_graph.dart';
+
+/// This file's composition root (see test_graph.dart).
+final _graph = testGraph();
+
 // The wheel anywhere on the full player flashes the volume readout, in
 // both skins, windowed and fullscreen, with the controls up or hidden.
 // The picture is a black box with the session's controls on it (never a
 // media_kit `Video`).
 
 void main() {
-  final coord = SessionState.instance.coordinator;
+  final coord = _graph.coordinator;
   final s = AppSettings.instance;
 
   // A real player for the volume steps to go to. Never disposed: the
@@ -46,7 +50,7 @@ void main() {
       for (final fullscreen in [false, true]) {
         final what = '${skin.name}, ${fullscreen ? 'fullscreen' : 'windowed'}';
         s.controlsSkin.value = skin;
-        await tester.pumpWidget(MaterialApp(
+        await tester.pumpWidget(scoped(_graph, MaterialApp(
           theme: ThemeData.dark(),
           home: Scaffold(
             body: SessionPlayer(
@@ -68,7 +72,7 @@ void main() {
               ]),
             ),
           ),
-        ));
+        )));
         await tester.pump();
         expect(opacity(), 0, reason: '$what: nothing before a step');
 
@@ -91,7 +95,7 @@ void main() {
         expect(opacity(), 0, reason: '$what: gone after the second');
       }
     }
-    await tester.pumpWidget(const SizedBox());
+    await tester.pumpWidget(scoped(_graph, const SizedBox()));
     await tester.pump(const Duration(seconds: 5));
   });
 }