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/delivery_test.dart

@@ -7,6 +7,11 @@ import 'package:sync_relay_core/core.dart';
 import 'package:starsync/src/session/delivery.dart';
 import 'package:starsync/src/shell/delivery_line.dart';
 
+import 'test_graph.dart';
+
+/// This file's composition root (see test_graph.dart).
+final _graph = testGraph();
+
 MemberView _m(String id, String name, {double buffered = 30, bool host = false, int rate = 0}) =>
     MemberView(id: id, name: name, ready: true, inSet: true, buffered: buffered, host: host, rate: rate);
 
@@ -134,7 +139,7 @@ void main() {
     final r = _state(
         _relayed(const MediaRelay(source: 'm1', live: true, rate: 1200000, need: 2300000)), [foxyz, bob, ann]);
     final d = deliveryOf(r, selfId: 'm2')!;
-    await tester.pumpWidget(MaterialApp(
+    await tester.pumpWidget(scoped(_graph, MaterialApp(
       theme: ThemeData.dark(),
       home: Scaffold(
         body: Center(
@@ -147,7 +152,7 @@ void main() {
           ),
         ),
       ),
-    ));
+    )));
     await tester.pumpAndSettle();
     expect(find.text("Foxyz's upload is slower than the video"), findsOneWidget);
     expect(find.byKey(const ValueKey('headroom-bar')), findsOneWidget);