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/ATTRIBUTION.md

@@ -0,0 +1,13 @@
+# Stroke data
+
+The files in this folder derive from **KanjiVG** (<https://kanjivg.tagaini.net>),
+the release of 2026-07-14, © 2009–2013 Ulrich Apel, distributed under the
+Creative Commons Attribution-Share Alike 3.0 licence
+(<https://creativecommons.org/licenses/by-sa/3.0/>). They are distributed under
+the same licence. `LICENSE.txt` is KanjiVG's notice, which the app's licence
+list shows.
+
+`halkyon_learn_curator`'s `curate kanjivg` writes them (see its README): each
+character's strokes as KanjiVG draws them and the components KanjiVG splits it
+into, keyed by the code point of KanjiVG's entry, in one gzipped JSON file for
+every 256 code points. Nothing else is changed.