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/scenes/playbackAdapter.js

@@ -16,7 +16,7 @@ export function createScenePlaybackAdapter(director, defaultShotDurationSec) {
       director._renderSceneSelect();
       director._renderShotList();
       director._updateStatus(
-        t('setup.scenes.status.runningShot', {
+        t('Running {index}/{total}: {scene} / {shot}', {
           index: index + 1,
           total,
           scene: scene.title,
@@ -78,7 +78,7 @@ export function createScenePlaybackAdapter(director, defaultShotDurationSec) {
     },
     complete() {
       director._setProgress(1);
-      director._updateStatus(t('setup.scenes.status.runComplete'));
+      director._updateStatus(t('Scene run complete'));
       director._logEvent('scene_run_complete', {});
     },
   };