refactor(app): delayed deletions move into halkyon_app (ADR 0030, D24)

Learn's DelayedDeletions, unchanged, with its test, so that Train's
workouts can be deleted with an undo as Learn's items are. Learn's
deletions import it from halkyon_app now.

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/async/delayed_deletions_test.dart

@@ -0,0 +1,49 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/halkyon_app.dart';
+
+import '../support/eventually.dart';
+
+void main() {
+  late List<String> deleted;
+  late DelayedDeletions deletions;
+
+  setUp(() {
+    deleted = [];
+    deletions = DelayedDeletions(
+      (id) async => deleted.add(id),
+      undoWindow: const Duration(milliseconds: 10),
+    );
+  });
+  tearDown(() => deletions.dispose());
+
+  test('a held deletion waits past the time to undo, until it is let go: '
+      'then it takes its time to undo', () async {
+    deletions.hold('a');
+    await Future<void>.delayed(const Duration(milliseconds: 50));
+    expect(deletions.pending, {'a'});
+    expect(deleted, isEmpty);
+
+    deletions.delete('a');
+    expect(deletions.pending, {'a'});
+    await eventually(() => deletions.pending.isEmpty);
+    expect(deleted, ['a']);
+  });
+
+  test('a held deletion can be undone', () async {
+    deletions
+      ..hold('a')
+      ..undo('a');
+    expect(deletions.pending, isEmpty);
+    await Future<void>.delayed(const Duration(milliseconds: 50));
+    expect(deleted, isEmpty);
+  });
+
+  test('closing drops a held deletion: the item stays', () async {
+    deletions.hold('a');
+    await deletions.dispose();
+    await Future<void>.delayed(const Duration(milliseconds: 50));
+
+    expect(deleted, isEmpty);
+    deletions = DelayedDeletions((id) async {});
+  });
+}