G16: Korean interface (ko)

Add a Korean catalog (crates/ui-egui/src/i18n/ko.tsv, 4,151 rows) covering
every menu string, every tl! literal and the status and error messages, and
register `ko` in i18n::LANGUAGES (plural_none; every ko-* locale resolves).

- tests: korean_reads_as_korean, ko locale tags, ko in KEEPS_MENU_NAMES and
  COMPLETE_MESSAGES; is_cjk covers Hangul so the bundled-font glyph check
  leaves Hangul to installed fonts, as it does CJK without craft-fonts.
- Hangul is drawn with an installed system font on the desktop; the web
  build still needs a Korean UI face in craft-fonts.
- docs: localization-parity (Korean row, 12-language score 54% -> 63%,
  60-100 h), ROADMAP, README, development.md, gaps.md G16,
  target-app-parity.

Clean-room: written from the meaning of the English strings in ordinary
Korean vocabulary; no other product's Korean localisation was used.
AI-drafted; native-speaker review welcome.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/development.md

@@ -171,7 +171,7 @@ always use the English ids and labels, so agents and scripts never see translate
 Languages shipped: English (`en`, the source), Traditional Chinese (`zh-hant`, complete, in the vocabulary used
 in Taiwan; `zh-TW`, `zh-HK`, `zh-MO` and `zh-Hant-*` locales all resolve to it), Simplified Chinese (`zh-hans`,
 complete, in the vocabulary used in mainland China; `zh-CN`, `zh-SG`, `zh-Hans-*` and a bare `zh` resolve to it,
-so the two scripts never mix), Japanese (`ja`, complete), German (`de`, complete; `de-DE`, `de-AT`, `de-CH` and every
+so the two scripts never mix), Japanese (`ja`, complete), Korean (`ko`, complete; every `ko-*` locale resolves to it; a count takes no plural, and Hangul is drawn with an installed system font), German (`de`, complete; `de-DE`, `de-AT`, `de-CH` and every
 other `de-*` locale resolve to it), Spanish (`es`, complete, in neutral
 international Spanish; every `es-*` locale such as `es-ES`, `es-MX`, `es-AR` or `es-419` resolves to it), French (`fr`,
 complete; `fr-FR`, `fr-BE`, `fr-CA`, `fr-CH` and every other `fr-*` locale resolve to it), Italian (`it`,