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/fixtures/kanji-data-sample.json

@@ -1,30 +0,0 @@
-{
-  "一": {
-    "strokes": 1,
-    "grade": 1,
-    "freq": 2,
-    "jlpt_old": 4,
-    "jlpt_new": 5
-  },
-  "食": {
-    "strokes": 9,
-    "grade": 2,
-    "freq": 328,
-    "jlpt_old": 4,
-    "jlpt_new": 5
-  },
-  "亜": {
-    "strokes": 7,
-    "grade": 8,
-    "freq": 1509,
-    "jlpt_old": 1,
-    "jlpt_new": 1
-  },
-  "唖": {
-    "strokes": 10,
-    "grade": null,
-    "freq": null,
-    "jlpt_old": null,
-    "jlpt_new": null
-  }
-}