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/country_names_test.dart

@@ -0,0 +1,16 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/halkyon_app.dart';
+
+void main() {
+  test('a country is named in the language asked for, else in English, and '
+      'one without a name not at all', () async {
+    final names = MemoryCountryNames({
+      'CH': {'de': 'Schweiz', 'en': 'Switzerland'},
+    });
+
+    expect(await names.nameOf('CH', 'de'), 'Schweiz');
+    expect(await names.nameOf('ch', 'de'), 'Schweiz');
+    expect(await names.nameOf('CH', 'ja'), 'Switzerland');
+    expect(await names.nameOf('XX', 'en'), isNull);
+  });
+}