chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

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/learn/halkyon_learn_flutter/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));
+  }
+}