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/support/eventually.dart

@@ -0,0 +1,17 @@
+import 'package:flutter_test/flutter_test.dart';
+
+/// Waits until [condition] holds, checking every few milliseconds, and fails
+/// after [timeout]: for what follows a real timer, without betting on how
+/// fast a busy machine runs.
+Future<void> eventually(
+  bool Function() condition, {
+  Duration timeout = const Duration(seconds: 10),
+}) async {
+  final stopwatch = Stopwatch()..start();
+  while (!condition()) {
+    if (stopwatch.elapsed > timeout) {
+      fail('Still not so after $timeout.');
+    }
+    await Future<void>.delayed(const Duration(milliseconds: 10));
+  }
+}