i18n(ja): ship Japanese

- Register ja: CATALOGS, LOCALES, LOCALE_NAMES (日本語) and the package
  boundaries; ja and ja-JP tags select it.
- Japanese unit words follow the number directly, as in the catalog:
  formatQuantity writes 5分 and 1時間5分 (km and kt keep the space), and
  formatRelativeTime drops the space CLDR puts in 5 分前.
- Tests: locale tags, picker order, Intl output, durations and the
  ramp/exit/lane wording in Japanese.
- docs/translations/ja.md: style, units, plurals and glossary; ja added to
  TRANSLATORS.md and CURRENT-STATE.md.

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

src/ui/unitLabels.js

@@ -1,4 +1,4 @@
-import { formatNumber, msg, t } from '../i18n/index.js';
+import { formatNumber, getLocale, msg, t } from '../i18n/index.js';
 
 // Unit ids stay English in data, params and AI-tool readings; only the label
 // shown on the page is translated. dBZ and °C read the same in every language.
@@ -23,6 +23,17 @@ export function unitLabel(unit) {
   return Object.hasOwn(UNIT_LABELS, unit) ? t(UNIT_LABELS[unit]) : unit || '';
 }
 
+/**
+ * What goes between a number and its unit label: a no-break space, except
+ * before a Japanese unit word, which follows the number directly (5分, 1時間).
+ */
+export function quantitySeparator(label) {
+  return getLocale() === 'ja' &&
+    /^[\p{Script=Han}\p{Script=Hiragana}\p{Script=Katakana}]/u.test(label)
+    ? ''
+    : '\u00a0';
+}
+
 /**
  * A number and its unit that never break apart: "22,0 км/ч". Readings carry
  * no digit grouping ("1008 hPa"), as on weather charts.
@@ -30,7 +41,7 @@ export function unitLabel(unit) {
 export function formatQuantity(value, unit, options) {
   const label = unitLabel(unit);
   const number = formatNumber(value, { useGrouping: false, ...options });
-  return label ? `${number}\u00a0${label}` : number;
+  return label ? `${number}${quantitySeparator(label)}${label}` : number;
 }
 
 /**