feat(todo): the calendars linked to projects fetched by the device and kept for offline (ADR 0033, D39)

`CalendarCubit` follows the projects' calendar links: a calendar linked
is shown at once from the copy kept and fetched by the device over HTTPS
(`HttpCalendarFeeds`, `webcal://` as HTTPS), as the app opens, every hour
while it is open and when a list is pulled down; a fetch that fails, or
answers with something that is no calendar, keeps the last copy; a link
taken off, or its project gone, takes its events and its copy along. The
copies are files in the person's own folder (`calendars/<project>.ics`).
Nothing shows the events yet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/test/feature/calendar/calendar_cubit_test.dart

@@ -0,0 +1,136 @@
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_todo_flutter/src/feature/calendar/bloc/calendar_cubit.dart';
+import 'package:halkyon_todo_flutter/src/feature/calendar/data/calendar_feeds.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/project.dart';
+import 'package:halkyon_todo_flutter/src/feature/project/model/stored_project.dart';
+
+import '../../support/fake_calendar_feeds.dart';
+
+/// A calendar file with one event, [title], on 10 Nov 2026 at nine.
+String feed(String title) =>
+    'BEGIN:VCALENDAR\r\nVERSION:2.0\r\n'
+    'BEGIN:VEVENT\r\nUID:$title@test\r\nDTSTART:20261110T090000\r\n'
+    'DTEND:20261110T100000\r\nSUMMARY:$title\r\nEND:VEVENT\r\n'
+    'END:VCALENDAR\r\n';
+
+void main() {
+  final from = DateTime(2026, 11, 10);
+  final until = DateTime(2026, 11, 11);
+
+  StoredProject school({String? url = 'https://example.org/school.ics'}) =>
+      StoredProject('school', ProjectContent(name: 'School', calendarUrl: url));
+
+  group("a project's calendar on the device (ADR 0033, D39)", () {
+    late FakeCalendarFeeds feeds;
+    late MemoryCalendarStore store;
+    late CalendarCubit cubit;
+
+    setUp(() {
+      feeds = FakeCalendarFeeds();
+      store = MemoryCalendarStore();
+      cubit = CalendarCubit(feeds, store);
+    });
+
+    tearDown(() => cubit.close());
+
+    test(
+      'a project linked is fetched, its events then by their project',
+      () async {
+        feeds.served[Uri.parse('https://example.org/school.ics')] = feed(
+          'Maths',
+        );
+
+        await cubit.follow([
+          school(),
+          const StoredProject('home', ProjectContent(name: 'Home')),
+        ]);
+
+        final events = cubit.state.eventsIn(from, until);
+        expect(events.map((entry) => (entry.projectId, entry.event.title)), [
+          ('school', 'Maths'),
+        ]);
+        expect(await store.read('school'), feed('Maths'));
+      },
+    );
+
+    test(
+      'fetched once as long as its address stays; again on refresh',
+      () async {
+        feeds.served[Uri.parse('https://example.org/school.ics')] = feed(
+          'Maths',
+        );
+        await cubit.follow([school()]);
+        await cubit.follow([school()]);
+        expect(feeds.fetched, hasLength(1));
+
+        feeds.served[Uri.parse('https://example.org/school.ics')] = feed('Art');
+        await cubit.refresh();
+        expect(feeds.fetched, hasLength(2));
+        expect(
+          cubit.state.eventsIn(from, until).single.event.title,
+          'Art',
+        );
+      },
+    );
+
+    test(
+      'offline, the last copy kept is shown; a failed fetch keeps it',
+      () async {
+        await store.write('school', feed('Kept'));
+
+        await cubit.follow([school()]);
+
+        expect(cubit.state.eventsIn(from, until).single.event.title, 'Kept');
+        expect(cubit.state.failed, {'school'});
+      },
+    );
+
+    test('a page that is no calendar is not kept', () async {
+      await store.write('school', feed('Kept'));
+      feeds.served[Uri.parse('https://example.org/school.ics')] =
+          '<html>Sign in</html>';
+
+      await cubit.follow([school()]);
+
+      expect(cubit.state.eventsIn(from, until).single.event.title, 'Kept');
+      expect(await store.read('school'), feed('Kept'));
+      expect(cubit.state.failed, {'school'});
+    });
+
+    test('unlinked, or the project gone, its events and its copy go', () async {
+      feeds.served[Uri.parse('https://example.org/school.ics')] = feed('Maths');
+      await cubit.follow([school()]);
+
+      await cubit.follow([school(url: null)]);
+
+      expect(cubit.state.eventsIn(from, until), isEmpty);
+      expect(await store.read('school'), isNull);
+    });
+
+    test('a webcal address is fetched over HTTPS', () async {
+      feeds.served[Uri.parse('https://example.org/school.ics')] = feed('Maths');
+
+      await cubit.follow([school(url: 'webcal://example.org/school.ics')]);
+
+      expect(feeds.fetched, [Uri.parse('https://example.org/school.ics')]);
+    });
+  });
+
+  test(
+    'the files kept in a folder of their own, read back and deleted',
+    () async {
+      final folder = await Directory.systemTemp.createTemp('calendars');
+      addTearDown(() => folder.delete(recursive: true));
+      final store = FileCalendarStore(folder);
+
+      expect(await store.read('school'), isNull);
+      await store.write('school', feed('Maths'));
+      expect(await store.read('school'), feed('Maths'));
+      await store.delete('school');
+      expect(await store.read('school'), isNull);
+      await store.delete('school');
+    },
+  );
+}