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/lib/src/app/app_scope.dart

@@ -0,0 +1,57 @@
+import 'package:flutter/widgets.dart';
+import 'package:halkyon_app/src/auth/data/auth_repository.dart';
+import 'package:halkyon_app/src/auth/data/signed_out_mark.dart';
+import 'package:halkyon_app/src/auth/model/auth_status.dart';
+
+/// What can be closed once it is no longer needed: an app's dependencies on
+/// an instance, or a person's.
+abstract interface class Disposable() {
+  /// Closes it; nothing of it is used afterwards.
+  Future<void> dispose();
+}
+
+/// What this package needs of an app's dependencies on one instance (its
+/// composition root), which `InstanceGate` opens for the instance in use and
+/// [AppScope] hands down. An app's own class implements it beside its
+/// features' repositories.
+abstract interface class AppDependencies<P extends Disposable>()
+    implements Disposable {
+  /// The instance's name as people know it (its host), for messages about
+  /// it.
+  String get serverName;
+
+  /// Who is signed in, and signing in and out.
+  AuthRepository get auth;
+
+  /// Whether the person here signed out, so the app asks how to start.
+  SignedOutMark get signedOut;
+
+  /// Opens the dependencies of [person], once the previous person's have
+  /// closed; `PersonScope` disposes them when the person goes.
+  Future<P> openPerson(SignedIn person);
+}
+
+/// Hands an app's [dependencies] on the instance in use down the widget
+/// tree; `InstanceGate` puts it above the app.
+class const AppScope({
+  /// The app's dependencies.
+  required final AppDependencies<Disposable> dependencies,
+  required super.child,
+  super.key,
+}) extends InheritedWidget {
+  /// The dependencies above [context], as the app's own type [D]
+  /// (`AppScope.of<Dependencies>(context)`), or as [AppDependencies] where
+  /// that is all that is needed.
+  ///
+  /// Doesn't register [context] for rebuilds: the dependencies of one app run
+  /// never change.
+  static D of<D extends AppDependencies<Disposable>>(BuildContext context) {
+    final scope = context.getInheritedWidgetOfExactType<AppScope>();
+    assert(scope != null, 'No AppScope above this widget.');
+    return scope!.dependencies as D;
+  }
+
+  @override
+  bool updateShouldNotify(AppScope oldWidget) =>
+      dependencies != oldWidget.dependencies;
+}