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>

halkyon_learn_curator/test/alphabets_test.dart

@@ -1,45 +0,0 @@
-import 'dart:convert';
-
-import 'package:halkyon_learn_curator/src/sources/alphabets.dart';
-import 'package:test/test.dart';
-
-void main() {
-  test('the Greek alphabet in its order, each letter with its capital, '
-      'names, transliteration and sound', () {
-    final [header, ...letters] = [
-      for (final line in greekAlphabet())
-        jsonDecode(line) as Map<String, Object?>,
-    ];
-
-    expect(header['source'], 'greek-alphabet');
-    expect(header['kind'], 'characters');
-    expect(header['language'], 'el');
-    expect(letters, hasLength(24));
-    expect(letters.first['headword'], 'Α α');
-    final sigma = letters.singleWhere((letter) => letter['id'] == 'σ');
-    expect(sigma['headword'], 'Σ σ ς');
-    expect(sigma['keys'], containsAll(['ς', 'σίγμα', 'sigma', 's']));
-    expect(letters[1]['entry'], {
-      'romaji': 'v',
-      'script': 'greek',
-      'name': 'βήτα',
-      'englishName': 'beta',
-      'sound': 'v',
-    });
-    // Found by a name typed without its accent.
-    expect(letters.last['keys'], contains('ωμεγα'));
-  });
-
-  test('the Hangul letters in the order schools teach them, found by sound '
-      'and name', () {
-    final letters = hangulEntries();
-
-    expect(letters, hasLength(40));
-    expect(letters.first.headword, 'ㄱ');
-    expect(letters.first.keys, containsAll(['ㄱ', '기역', 'g', 'k']));
-    // ㅇ is silent opening a syllable: no key for its sound there.
-    final ieung = letters.singleWhere((letter) => letter.id == 'ㅇ');
-    expect(ieung.keys, ['ㅇ', '이응', 'ng']);
-    expect(letters.last.headword, 'ㅢ');
-  });
-}