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/tool/maps/japan_prefectures.py

@@ -0,0 +1,69 @@
+#!/usr/bin/env python3
+"""Builds the app's map of Japan's prefectures from Natural Earth.
+
+Natural Earth's vector data is in the public domain. This script downloads
+its Admin 1 (states, provinces) layer at 1:10m, release v5.1.2, checks the
+download against the hash recorded below, keeps Japan's 47 prefectures, and
+reduces them to a compact outline for display (outlines.py): Douglas-Peucker
+simplification (0.006 degrees), islands too small to see dropped (each
+prefecture keeps its largest), coordinates rounded to 4 decimals. The result
+is an approximation, not survey-accurate data.
+
+Each region is named by its ISO 3166-2 code (JP-01 ... JP-47), with Natural
+Earth's English and Japanese names.
+
+Run from the repository root with Python 3 and no other packages:
+
+    python3 tool/maps/japan_prefectures.py
+
+It rewrites halkyon_learn_flutter/assets/maps/japan_prefectures.geojson.
+"""
+
+import json
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent))
+from outlines import download, polygons_of, reduce  # noqa: E402
+
+SOURCE = (
+    "https://raw.githubusercontent.com/nvkelso/natural-earth-vector/"
+    "v5.1.2/geojson/ne_10m_admin_1_states_provinces.geojson"
+)
+SOURCE_SHA256 = "22d0e3ad85eb3e27f17cabf8ba2d50e554fbc27a87796ff891d958185da62fb5"
+TARGET = Path("halkyon_learn_flutter/assets/maps/japan_prefectures.geojson")
+
+TOLERANCE = 0.006  # degrees
+MIN_AREA = 0.0005  # square degrees; smaller islands are dropped
+DECIMALS = 4
+
+
+def main():
+    source = json.loads(download(SOURCE, SOURCE_SHA256))
+    features = []
+    for feature in source["features"]:
+        properties = feature["properties"]
+        if properties.get("adm0_a3") != "JPN":
+            continue
+        features.append({
+            "type": "Feature",
+            "properties": {
+                "code": properties["iso_3166_2"],
+                "name": properties["name"],
+                "name_ja": properties["name_ja"],
+            },
+            "geometry": reduce(polygons_of(feature["geometry"]), TOLERANCE,
+                               MIN_AREA, DECIMALS),
+        })
+    features.sort(key=lambda feature: feature["properties"]["code"])
+    codes = [feature["properties"]["code"] for feature in features]
+    if codes != [f"JP-{n:02d}" for n in range(1, 48)]:
+        sys.exit(f"Expected the 47 prefectures JP-01 ... JP-47, got {codes}")
+    TARGET.write_text(json.dumps(
+        {"type": "FeatureCollection", "features": features},
+        ensure_ascii=False, separators=(",", ":")) + "\n", encoding="utf-8")
+    print(f"Wrote {TARGET} ({TARGET.stat().st_size} bytes)", file=sys.stderr)
+
+
+if __name__ == "__main__":
+    main()