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/app/scene.js

@@ -43,7 +43,7 @@ export async function createApplicationScene({
       else window.__GOOGLE_MAPS_API_KEY__ = previousKey;
     });
   }
-  loaderStatus.textContent = t('shell.loading.status.configuring');
+  loaderStatus.textContent = t('Configuring viewer...');
   // Provider attribution stays visible, including clean-view and recording.
   const creditContainer = document.createElement('div');
   creditContainer.id = 'cesium-credits';
@@ -62,8 +62,8 @@ export async function createApplicationScene({
   configureCreditKeyboardAccess(document);
   loaderStatus.textContent =
     googleApiKey || googleTokens || cesiumToken
-      ? t('shell.loading.status.tilesGoogle')
-      : t('shell.loading.status.tilesKeyless');
+      ? t('Loading Google 3D Tiles...')
+      : t('Loading the keyless globe...');
   const photoreal = await loadPhotorealisticTileset(Cesium, {
     googleApiKey,
     googleTokens,
@@ -91,14 +91,17 @@ export async function createApplicationScene({
         tileError,
       );
       const tileErrorDetail = describeError(tileError);
-      loaderStatus.textContent = t('shell.loading.status.tilesUnavailable', {
-        detail: tileErrorDetail,
-      });
+      loaderStatus.textContent = t(
+        'Google 3D Tiles unavailable ({detail}). Loading the keyless globe...',
+        {
+          detail: tileErrorDetail,
+        },
+      );
     }
     viewer.scene.globe.show = true;
   }
 
-  loaderStatus.textContent = t('shell.loading.status.systems');
+  loaderStatus.textContent = t('Initializing systems...');
 
   const mapStackController = new MapController(viewer, {
     requestRender: governorRequestRender,