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]>

src/i18n/i18n.test.mjs

@@ -35,6 +35,12 @@ test('normalizeLocale folds regional variants and rejects unshipped tags', () =>
   assert.equal(normalizeLocale(' RU '), 'ru');
   assert.equal(normalizeLocale('en_GB'), 'en');
   assert.equal(normalizeLocale('xx'), null);
+  // Simplified Chinese by default; Traditional tags wait for zh-TW.
+  assert.equal(normalizeLocale('zh'), 'zh-CN');
+  assert.equal(normalizeLocale('zh-CN'), 'zh-CN');
+  assert.equal(normalizeLocale('zh-Hans-SG'), 'zh-CN');
+  assert.equal(normalizeLocale('zh-TW'), null);
+  assert.equal(normalizeLocale('zh-Hant-HK'), null);
   assert.equal(normalizeLocale(undefined), null);
 });