refactor(i18n): key messages by English text; add Russian

Replace the semantic keys with the English source text, so code reads as the
UI does and English needs no catalog: t('Save keys'), tn(count, one, other)
for plurals, msg() for text stored in data, and bare data-i18n* markup that
translates the element's own English. Other languages are one flat JSON each.

- Simplify src/i18n: drop namespaces, the configurable locale pair
  (GEV_DEFAULT_LOCALE / GEV_SECONDARY_LOCALE) and its build defines. Locale
  comes from ?lang=, the stored choice, then browser languages.
- Add scripts/i18n.mjs (npm run i18n): extracts every message from t/tn/msg
  and markup, and fails on missing, unused, placeholder-mismatched or
  wrongly-shaped plural entries. npm test runs it via catalog.test.mjs.
- Add Russian (src/i18n/locales/ru.json, 833 messages), started from the
  community catalog in #312 and reviewed against the current English.
- Fix data-i18n placed on parents that would wipe their children (cockpit
  LEVEL and local weather cells, first-run EXPLORE MANUALLY tile), and a
  doubled degree sign in the TRACK ACQUIRED signal.
- Rewrite docs/TRANSLATORS.md for the new workflow; drop sections the earlier
  merge resurrected in docs/CURRENT-STATE.md.

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

src/layers/awareness/rendering.js

@@ -138,7 +138,7 @@ export function createRendering({
       '--compass-rotation',
       `${-headingDeg}deg`,
     );
-    layerState.compassHeading.textContent = t('layers.awareness.hdg', {
+    layerState.compassHeading.textContent = t('HDG {value}°', {
       value: String(headingDeg).padStart(3, '0'),
     });
     layerState.compassHeading.style.left = `${geometry.centerX}px`;
@@ -224,12 +224,12 @@ export function createRendering({
             )
           : null;
       const bearingText = Number.isFinite(bearing)
-        ? t('layers.awareness.brg', {
+        ? t('BRG {value}°', {
             value: String(Math.round(bearing)).padStart(3, '0'),
           })
-        : t('layers.awareness.brgUnknown');
+        : t('BRG —');
       const courseText = Number.isFinite(item.track)
-        ? ` · ${t('layers.awareness.crs', {
+        ? ` · ${t('CRS {value}°', {
             value: String(Math.round(item.track)).padStart(3, '0'),
           })}`
         : '';