hanja: add single-character dictionary data

map/README

@@ -1,4 +1,44 @@
-# Japanese dictionary data
+# Dictionary data
+
+## Korean Hanja data
+
+`hanja.src` is the tracked, reviewable source for Korean Hanja conversion.
+Every data row is exactly one Hanja character, a tab, and one modern Hangul
+syllable:
+
+```
+漢	한
+```
+
+It contains no word rows such as `견출지` or `방학`.  `mkhanja` validates this
+contract and groups rows by their Hangul reading to produce the existing
+runtime dictionary format.  Candidate order follows source order.  The source
+keeps all retained pairs for review; the generated dictionary stores the first
+32 candidates per reading because that is the engine's lookup limit.
+
+The source is derived from libhangul release tag `libhangul-0.2.0`.  The
+annotated tag object is `20afc38922e3595ee3ed5b186f2ea05afe663763`, its
+peeled commit is `41c702f5d3581325b646ef6249f1f641b0427ae0`, and
+`data/hanja/hanja.txt` has blob ID
+`199cfd70c4b306257ac2e018714a1285f7cf0ed3`.  Import and regenerate it from
+the repository root with:
+
+```sh
+curl -L https://raw.githubusercontent.com/libhangul/libhangul/41c702f5d3581325b646ef6249f1f641b0427ae0/data/hanja/hanja.txt -o upstream
+printf '%s  %s\n' dd44dcc856cf542b1022d0f39c2e9b9f8805fdcc5923be80f04849ed97ce0996 upstream | sha256sum -c -
+map/libhangul2hanja upstream >map/hanja.src
+map/mkhanja >map/hanja.dict
+```
+
+`libhangul2hanja` retains only rows whose reading is one modern Hangul
+syllable and whose value is one Hanja character supported by the current
+popup: U+3400–U+4DBF, U+4E00–U+9FFF, or U+F900–U+FAFF.  Thus word readings,
+multi-character values, jamo readings, and supplementary-plane ideographs
+are omitted.  The import preserves the upstream license header and row order.
+The retained data is BSD 3-Clause licensed by Choe Hwanjin; the complete text
+is in `LICENSES/BSD-3-Clause-libhangul-hanja.txt`.
+
+## Japanese dictionary data
 
 `kanji.dict` is the historical dictionary bundled with strans.  Its own header
 identifies it as the SKK Medium dictionary, version 8.1 of May 24, 1995,