halkyon_app: instances, sign-in and sync plumbing for Halkyon's apps

What every Halkyon app's shell needs, taken from Halkyon Learn's and set
on halkyon_base's client: the instances a device keeps an account on,
the first start's tiles and where an instance keeps people's data, the
account switcher; signing in anonymously, with GitHub or an email code,
switching person and the start screen; the gates that build an app's
dependencies per instance and per person; and the sync service with its
status, Settings card and banner. Texts of its own in English and
German take the app's name; what only the app can say, it passes in.
Fakes for apps' tests are in testing.dart.

An instance's basics now name the app it serves (`app`, set by the
host's configuration), and an app accepts only an instance of its own.
SerialQueue and CoalescingRunner move into halkyon_core.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/test/app/person_scope_test.dart

@@ -0,0 +1,55 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/halkyon_app.dart';
+import 'package:halkyon_app/testing.dart';
+import 'package:material_ui/material_ui.dart';
+
+/// What the person sees: whose dependencies are open above it.
+class const _Person() extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) =>
+      Text('Data of ${PersonScope.of<TestPerson>(context).person.authUserId}');
+}
+
+void main() {
+  const ada = SignedIn(authUserId: 'ada', isIdentified: true);
+  late TestAppDependencies dependencies;
+
+  setUp(() => dependencies = TestAppDependencies());
+
+  Future<void> show(WidgetTester tester, Widget scope) async {
+    await tester.pumpWidget(testApp(scope, dependencies: dependencies));
+    await tester.pumpAndSettle();
+  }
+
+  testWidgets("shows the person's app once their data is open", (
+    tester,
+  ) async {
+    await show(tester, const PersonScope(person: ada, child: _Person()));
+
+    expect(find.text('Data of ada'), findsOneWidget);
+    expect(dependencies.opened.single.person, ada);
+  });
+
+  testWidgets("says so when the person's data can't be opened, and offers "
+      'to try again; an attempt that works shows the app', (tester) async {
+    dependencies.openingError = Exception('database locked');
+    await show(tester, const PersonScope(person: ada, child: _Person()));
+
+    expect(find.text("Couldn't open this device's data."), findsOneWidget);
+    expect(find.text('Data of ada'), findsNothing);
+
+    dependencies.openingError = null;
+    await tester.tap(find.text('Try again'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Data of ada'), findsOneWidget);
+  });
+
+  testWidgets("closes the person's data when it goes away", (tester) async {
+    await show(tester, const PersonScope(person: ada, child: _Person()));
+
+    await show(tester, const Text('Someone else'));
+
+    expect(dependencies.opened.single.disposed, isTrue);
+  });
+}