i18n(zh-CN): complete and ship Simplified Chinese

- Translate the remaining 910 strings and review the whole catalog (meaning,
  terms, punctuation, label length) against the screens.
- Register zh-CN: any zh tag maps to it except Traditional ones (Hant, TW,
  HK, MO), which keep English until a Traditional catalog ships.
- Hemisphere letters get their own context so the HUD sector line reads
  21.33N, not a compass word; the Bhote Koshi imagery dates follow
  year-first languages (2021年10月16日).
- cjk.css: tight tracking for Chinese text and a 9px floor on the dock voice
  labels and layer buttons; Latin HUD readouts keep their look.
- docs/translations/zh-CN.md: style, plurals and glossary.

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

docs/TRANSLATORS.md

@@ -1,6 +1,6 @@
 # Translating God's Eye View
 
-Shipped languages: **English** (source and fallback), **Spanish** (`es`) and **Russian** (`ru`).
+Shipped languages: **English** (source and fallback), **Spanish** (`es`), **Russian** (`ru`) and **Simplified Chinese** (`zh-CN`).
 
 ## How it works
 
@@ -63,4 +63,4 @@ These are contracts, not copy: layer ids and registry keys, status enums and oth
 
 ## Language notes
 
-Each language keeps its glossary, plural forms, units and style in its own file, `docs/translations/<code>.md`: [Spanish](./translations/es.md), [Russian](./translations/ru.md). Read it before translating, and add a term there when you settle on one.
+Each language keeps its glossary, plural forms, units and style in its own file, `docs/translations/<code>.md`: [Spanish](./translations/es.md), [Russian](./translations/ru.md), [Simplified Chinese](./translations/zh-CN.md). Read it before translating, and add a term there when you settle on one.