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/maps/ATTRIBUTION.md

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+# Map data
+
+Both maps derive from **Natural Earth** (<https://www.naturalearthdata.com/>),
+release v5.1.2. Natural Earth's map data is in the public domain:
+
+> All versions of Natural Earth raster and vector map data found on this
+> website are in the public domain. You may use the maps in any manner,
+> including modifying the content and design, electronic dissemination, and
+> offset printing.
+
+## Japan's prefectures
+
+`japan_prefectures.geojson` derives from *Admin 1 – States, Provinces*, 1:10m
+cultural vectors. `tool/maps/japan_prefectures.py` builds the file: it keeps
+Japan's 47 prefectures, simplifies their outlines (Douglas–Peucker, 0.006°),
+drops islands too small to see (each prefecture keeps its largest), and rounds
+coordinates to 4 decimals. The outlines are an approximation for display, not
+survey-accurate data. Regions are named by their ISO 3166-2 codes (`JP-01` …
+`JP-47`), with Natural Earth's English and Japanese names.
+
+## Countries of the world
+
+`world_countries.geojson` derives from *Admin 0 – Countries*, 1:50m cultural
+vectors (Gibraltar and the US Outlying Islands, which it leaves out, from the
+1:10m layer). `tool/maps/world_countries.py` builds the file: one region per
+ISO 3166-1 code (239), areas without a code of their own joined to the country
+ISO counts them under, outlines kept as Natural Earth generalised them (islands
+under 0.001 square degrees dropped), coordinates rounded to 3 decimals. Borders
+are drawn as Natural Earth draws them, as they are on the ground.
+
+The countries' names come from the **Unicode Common Locale Data Repository**
+(CLDR, <https://cldr.unicode.org/>), release 48.2.3, in every language a
+studyset can be in. CLDR is under the Unicode License v3, whose notice is in
+`LICENSE-CLDR.txt` and in the app's licence list.

apps/learn/halkyon_learn_flutter/assets/maps/LICENSE-CLDR.txt

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+UNICODE LICENSE V3
+
+COPYRIGHT AND PERMISSION NOTICE
+
+Copyright © 2015-2024 Unicode, Inc.
+
+NOTICE TO USER: Carefully read the following legal agreement. BY
+DOWNLOADING, INSTALLING, COPYING OR OTHERWISE USING DATA FILES, AND/OR
+SOFTWARE, YOU UNEQUIVOCALLY ACCEPT, AND AGREE TO BE BOUND BY, ALL OF THE
+TERMS AND CONDITIONS OF THIS AGREEMENT. IF YOU DO NOT AGREE, DO NOT
+DOWNLOAD, INSTALL, COPY, DISTRIBUTE OR USE THE DATA FILES OR SOFTWARE.
+
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+Data Files or Software are furnished to do so, provided that either (a)
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+
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