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/assets/kanjivg/LICENSE.txt

@@ -0,0 +1,18 @@
+The stroke order data of the kanji and kana is KanjiVG's:
+https://kanjivg.tagaini.net
+
+Copyright (C) 2009-2013 Ulrich Apel.
+This work is distributed under the conditions of the Creative Commons
+Attribution-Share Alike 3.0 Licence. This means you are free:
+* to Share - to copy, distribute and transmit the work
+* to Remix - to adapt the work
+
+Under the following conditions:
+* Attribution. You must attribute the work by stating your use of KanjiVG in
+  your own copyright header and linking to KanjiVG's website
+  (http://kanjivg.tagaini.net)
+* Share Alike. If you alter, transform, or build upon this work, you may
+  distribute the resulting work only under the same or similar license to this
+  one.
+
+See http://creativecommons.org/licenses/by-sa/3.0/ for more details.