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/fonts/ATTRIBUTION.md

@@ -0,0 +1,19 @@
+# Fonts
+
+The app brings its fonts with it and never fetches any: text looks the same on
+every device, whatever fonts it has. Noto Sans, its font, comes with halkyon_ui
+(halkyon_shared), which bundles it with its licence and registers it; the fonts
+for Japanese, Chinese and Korean are the app's own, here.
+
+| File | Font | Source | Licence |
+|---|---|---|---|
+| `NotoSansJP-Variable.otf`, `NotoSansSC-Variable.otf`, `NotoSansTC-Variable.otf`, `NotoSansKR-Variable.otf` | Noto Sans JP, SC, TC, KR (Japanese, Simplified and Traditional Chinese, Korean), every weight | [notofonts/noto-cjk](https://github.com/notofonts/noto-cjk) `Sans/Variable/OTF/Subset/NotoSans{JP,SC,TC,KR}-VF.otf`, commit f8d157532fbf (2026-10-08) | SIL Open Font License 1.1, `OFL-NotoSansCJK.txt` |
+
+Noto Sans is the app's font; the CJK fonts fill in the glyphs it lacks, the
+one matching a studyset's language first, Japanese otherwise (halkyon_ui's
+`appFontFallbackFor`, which names them by the families `pubspec.yaml`
+declares): they share most Han characters but draw them differently. Noto Sans JP alone would not do as the app's font: it has
+no accented or polytonic Greek. Scripts none of them covers (Arabic, Hebrew,
+Thai, Devanagari, …) come from the device's own fonts. The files are renamed
+only; their glyphs are unchanged. The licences are shown in the app's licence
+list.