feat(app): stroke order for kanji and kana, from KanjiVG

KanjiVG's strokes come with the app (assets/kanjivg/, CC BY-SA 3.0, its
notice in the licence list): 94 gzipped files, one per 256 code points,
2.7 MB in all, a file read the first time a character of it is asked for
(StrokeRepository, app-wide beside the maps). The strokes are SVG path
data, read with path_parsing (the Flutter team's, pure Dart, already in
the build through flutter_svg), so the model can flatten them into points
without dart:ui, as checking handwriting will.

A character's sheet traces its strokes in order over those to come, with
its stroke count, meanings, readings, level and the components KanjiVG
splits it into, each a tap away. It opens from a studied character's
answer (Stroke order) and from a character found in the dictionary.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

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.