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/base/base_client_test.dart

@@ -0,0 +1,106 @@
+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';
+
+/// [refusal] as an instance sends it: named by its module.
+String sent(SerializableException refusal) => jsonEncode({
+  'className':
+      'halkyon_base.${switch (refusal) {
+        EmailSignInRefusedException() => 'EmailSignInRefusedException',
+        GitHubSignInRefusedException() => 'GitHubSignInRefusedException',
+        _ => throw ArgumentError.value(refusal),
+      }}',
+  'data': (refusal as SerializableModel).toJson(),
+});
+
+void main() {
+  test("reads the refusals an instance sends under halkyon_base's name, as "
+      "an app's own generated protocol does", () {
+    final protocol = BaseProtocol();
+
+    expect(
+      protocol.decodeWithType(
+        sent(
+          EmailSignInRefusedException(reason: EmailSignInRefusal.wrongCode),
+        ),
+      ),
+      isA<EmailSignInRefusedException>().having(
+        (refusal) => refusal.reason,
+        'reason',
+        EmailSignInRefusal.wrongCode,
+      ),
+    );
+    expect(
+      protocol.decodeWithType(
+        sent(
+          GitHubSignInRefusedException(
+            reason: GitHubSignInRefusal.unknownAccount,
+          ),
+        ),
+      ),
+      isA<GitHubSignInRefusedException>().having(
+        (refusal) => refusal.reason,
+        'reason',
+        GitHubSignInRefusal.unknownAccount,
+      ),
+    );
+  });
+
+  test("calls halkyon_base's endpoints at the instance, and passes its "
+      'refusals on as themselves', () async {
+    final asked = <String>[];
+    final client = BaseClient(
+      'https://learn.example.org/',
+      httpClientOverride: MockClient((request) async {
+        asked.add(request.url.toString());
+        return switch (request.url.path) {
+          '/halkyon_base.instance/basics' => http.Response(
+            jsonEncode(
+              InstanceBasics(
+                app: 'learn',
+                emailSignIn: true,
+                sourceCodeUrl: 'https://code.example.org/learn',
+                minimumAppBuild: 21,
+                dataLocation: 'EU',
+              ).toJson(),
+            ),
+            200,
+          ),
+          '/halkyon_base.emailSignIn/signIn' => http.Response(
+            sent(
+              EmailSignInRefusedException(
+                reason: EmailSignInRefusal.codeExpired,
+              ),
+            ),
+            400,
+          ),
+          _ => http.Response('', 404),
+        };
+      }),
+    );
+    addTearDown(client.close);
+
+    final basics = await client.modules.instance.basics();
+    expect(basics.emailSignIn, isTrue);
+    expect(basics.minimumAppBuild, 21);
+    expect(basics.dataLocation, 'EU');
+    await expectLater(
+      client.modules.emailSignIn.signIn(switchToken: 'token'),
+      throwsA(
+        isA<EmailSignInRefusedException>().having(
+          (refusal) => refusal.reason,
+          'reason',
+          EmailSignInRefusal.codeExpired,
+        ),
+      ),
+    );
+    expect(asked, [
+      'https://learn.example.org/halkyon_base.instance/basics',
+      'https://learn.example.org/halkyon_base.emailSignIn/signIn',
+    ]);
+  });
+}