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/mapSourceFocus.test.mjs

@@ -5,7 +5,7 @@ import { createHoverDisclosure, collapsePanelOnEscape } from './ui/panelDisclosu
 import test from 'node:test';
 import assert from 'node:assert/strict';
 import { readFileSync } from 'node:fs';
-import { t } from './i18n/index.js';
+import { msg, t } from './i18n/index.js';
 
 // Exercise the installed event routes and central close method, without WebGL.
 const source = readShellSource();
@@ -108,9 +108,9 @@ function harness({ hidden = false, selected = true, noChips = false } = {}) {
   window.clearTimeout = (id) => timers.delete(id);
   window.performance = { now: () => now };
   document.defaultView = window;
-  const methods = new Function('createHoverDisclosure', 'collapsePanelOnEscape', 'document', 'window', 'clearTimeout', 'performance', 'requestAnimationFrame', 't',
+  const methods = new Function('createHoverDisclosure', 'collapsePanelOnEscape', 'document', 'window', 'clearTimeout', 'performance', 'requestAnimationFrame', 't', 'msg',
     `return ({${shellMethod('_initAutoHoverPanel').toString()},\n${shellMethod('_collapsePanelOnEscape').toString()},\n${shellMethod('setPanelCollapsed').toString()},\n${shellMethod('_syncPanelCollapseButton').toString()}});`)(
-    createHoverDisclosure, collapsePanelOnEscape, document, window, (id) => timers.delete(id), { now: () => now }, () => {}, t,
+    createHoverDisclosure, collapsePanelOnEscape, document, window, (id) => timers.delete(id), { now: () => now }, () => {}, t, msg,
   );
   const saves = [];
   const claims = [];