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/instance/instance_checker_test.dart

@@ -0,0 +1,67 @@
+import 'dart:convert';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/halkyon_app.dart';
+import 'package:halkyon_base_client/halkyon_base_client.dart';
+import 'package:http/http.dart' as http;
+import 'package:http/testing.dart';
+
+void main() {
+  final halkyon = Instance.parse('learn.example.org')!;
+  final todo = Instance.parse('todo.example.org')!;
+  final elsewhere = Instance.parse('www.example.org')!;
+
+  /// Basics as an instance of [app] tells them.
+  http.Response basicsOf(String app) => http.Response(
+    jsonEncode(
+      InstanceBasics(
+        app: app,
+        emailSignIn: false,
+        sourceCodeUrl: 'https://code.example.org/$app',
+        minimumAppBuild: 1,
+        dataLocation: 'EU',
+        backupLocation: 'CH',
+      ).toJson(),
+    ),
+    200,
+  );
+
+  /// A checker of the app `learn` reaching instances through a fake network,
+  /// where learn.example.org answers as an instance of it, todo.example.org
+  /// as an instance of another app, and nothing else answers.
+  ServerpodInstanceChecker checker() => ServerpodInstanceChecker(
+    app: 'learn',
+    httpClient: () => MockClient(
+      (request) async => switch ((request.url.host, request.url.path)) {
+        ('learn.example.org', '/halkyon_base.instance/basics') => basicsOf(
+          'learn',
+        ),
+        ('todo.example.org', '/halkyon_base.instance/basics') => basicsOf(
+          'todo',
+        ),
+        _ => http.Response('<html>Not found</html>', 404),
+      },
+    ),
+  );
+
+  test('an instance answers when its halkyon_base tells its basics; any '
+      'other site does not', () async {
+    expect(await checker().answers(halkyon), isTrue);
+    expect(await checker().answers(elsewhere), isFalse);
+  });
+
+  test("another app's instance does not answer, so its address is refused "
+      'before the app moves there', () async {
+    expect(await checker().answers(todo), isFalse);
+    expect(await checker().locationsOf(todo), isNull);
+  });
+
+  test("says where an instance keeps people's data and its backups, and "
+      'nothing for a site that is no instance', () async {
+    expect(
+      await checker().locationsOf(halkyon),
+      const InstanceLocations(data: 'EU', backups: 'CH'),
+    );
+    expect(await checker().locationsOf(elsewhere), isNull);
+  });
+}