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/hud.js

@@ -202,15 +202,15 @@ export class IntelHUD {
           <div class="hud-system">${this._missionId}  ${this._sensorId}</div>
           <div class="hud-mode" id="hud-mode">NORMAL</div>
           <div class="hud-summary-wrap">
-            <div class="hud-summary-label">${t('cockpit.hud.summaryLabel')}</div>
-            <div class="hud-summary" id="hud-summary">${t('cockpit.hud.summaryAwaiting')}</div>
+            <div class="hud-summary-label">${t('SUMMARY')}</div>
+            <div class="hud-summary" id="hud-summary">${t('Awaiting telemetry...')}</div>
           </div>
         </div>
       </div>
 
       <div class="hud-corner hud-top-right">
         <div class="hud-content" style="text-align:right">
-          <div class="hud-rec"><span id="hud-rec-dot">●</span> ${t('cockpit.hud.recLabel')}  <span id="hud-timestamp">2026-01-01 00:00:00Z</span></div>
+          <div class="hud-rec"><span id="hud-rec-dot">●</span> ${t('REC')}  <span id="hud-timestamp">2026-01-01 00:00:00Z</span></div>
           <div class="hud-orbital">ORB: ${this._orbitNum}  PASS: DESC-${this._passNum}</div>
         </div>
         <div class="hud-bracket">┐</div>
@@ -635,7 +635,7 @@ export class IntelHUD {
    */
   _composeSummary() {
     const m = this._latestMetrics;
-    if (!m) return t('cockpit.hud.summaryAwaiting');
+    if (!m) return t('Awaiting telemetry...');
 
     const modeEl = document.getElementById('hud-mode');
     const modeLabel = modeEl?.textContent || 'NORMAL';