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_test.dart

@@ -0,0 +1,51 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/halkyon_app.dart';
+
+void main() {
+  test('reads an address as people type it, HTTPS unless they say not', () {
+    for (final address in [
+      'learn.example.org',
+      'https://learn.example.org',
+      ' HTTPS://Learn.Example.org/ ',
+    ]) {
+      expect(
+        Instance.parse(address)?.url,
+        Uri.parse('https://learn.example.org/'),
+        reason: address,
+      );
+    }
+    expect(
+      Instance.parse('http://localhost:8080')?.url,
+      Uri.parse('http://localhost:8080/'),
+    );
+    expect(
+      Instance.parse('example.org/learn?x=1#top')?.url,
+      Uri.parse('https://example.org/learn/'),
+    );
+  });
+
+  test('refuses what is no address', () {
+    for (final address in [
+      '',
+      '  ',
+      'learn example org',
+      'ftp://example.org',
+      'https://',
+      'https://someone@example.org',
+    ]) {
+      expect(Instance.parse(address), isNull, reason: address);
+    }
+  });
+
+  test('is named by its host, and its port if it has one', () {
+    expect(Instance.parse('learn.halkyon.eu')!.name, 'learn.halkyon.eu');
+    expect(Instance.parse('localhost:8080')!.name, 'localhost:8080');
+  });
+
+  test('is the same instance however its address was written', () {
+    expect(
+      Instance.parse('learn.example.org'),
+      Instance.parse('https://LEARN.example.org/'),
+    );
+  });
+}