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/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.