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_curator/test/kana_test.dart

@@ -0,0 +1,37 @@
+import 'package:halkyon_learn_curator/src/japanese/kana.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('katakana becomes hiragana; the rest stays', () {
+    expect(toHiragana('テレビ'), 'てれび');
+    expect(toHiragana('カード'), 'かーど');
+    expect(toHiragana('食べる'), '食べる');
+  });
+
+  test('kana become Hepburn romaji, as people type them', () {
+    expect(toRomaji('たべる'), 'taberu');
+    expect(toRomaji('とうきょう'), 'toukyou');
+    expect(toRomaji('しゃしん'), 'shashin');
+    expect(toRomaji('きって'), 'kitte');
+    expect(toRomaji('まっちゃ'), 'matcha');
+    expect(toRomaji('カード'), 'kaado');
+    expect(toRomaji('パーティー'), 'paatii');
+    expect(toRomaji('ふぁいる'), 'fairu');
+    expect(toRomaji('ぢ'), 'ji');
+    expect(toRomaji('デュエット'), 'dyuetto');
+    expect(toRomaji('フュージョン'), 'fyuujon');
+  });
+
+  test('long vowels written once, where that differs', () {
+    expect(withShortVowels('toukyou'), 'tokyo');
+    expect(withShortVowels('kaado'), 'kado');
+    expect(withShortVowels('oniisan'), 'onisan');
+    expect(withShortVowels('taberu'), isNull);
+  });
+
+  test('full-width letters and digits become the ones people type', () {
+    expect(halfWidth('Ｔシャツ'), 'Tシャツ');
+    expect(halfWidth('３Ｄ　ＣＤ'), '3D CD');
+    expect(halfWidth('東京'), '東京');
+  });
+}