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/traffic/model.js

@@ -261,12 +261,12 @@ export function createModel({ state: layerState, services, parts, source }) {
         mode,
         error: null,
         loadingLabel: fetching
-          ? t('layers.traffic.syncingRoads', { source: roadSource })
+          ? t('Syncing flow · Roads: {source}', { source: roadSource })
           : coveragePct > 0
-            ? t('layers.traffic.liveRoadsFlowHidden', {
+            ? t('LIVE · Roads: {source} · Roads without flow hidden', {
                 source: roadSource,
               })
-            : t('layers.traffic.liveRoadsNoFlowInView', {
+            : t('LIVE · Roads: {source} · No flow roads in view', {
                 source: roadSource,
               }),
       };
@@ -276,13 +276,13 @@ export function createModel({ state: layerState, services, parts, source }) {
         mode,
         error: null,
         loadingLabel: fetching
-          ? t('layers.traffic.syncingRoads', { source: roadSource })
+          ? t('Syncing flow · Roads: {source}', { source: roadSource })
           : coveragePct > 0
-            ? t('layers.traffic.roadsFlowLive', {
+            ? t('LIVE · Roads: {source} · Flow {percent}%', {
                 source: roadSource,
                 percent: coveragePct,
               })
-            : t('layers.traffic.roadsFlowSimulated', {
+            : t('SIMULATED · Roads: {source} · Flow {percent}%', {
                 source: roadSource,
                 percent: coveragePct,
               }),
@@ -293,16 +293,24 @@ export function createModel({ state: layerState, services, parts, source }) {
         mode,
         error: null,
         loadingLabel: fetching
-          ? t('layers.traffic.loadingSyncing')
+          ? t(
+              'Syncing flow · Roads: OpenStreetMap · Flow: TomTom · Unmatched: simulated',
+            )
           : coveragePct > 0
             ? coveragePct < 100
-              ? t('layers.traffic.liveCoverageUnmatched', {
-                  percent: coveragePct,
-                })
-              : t('layers.traffic.liveCoverage', {
-                  percent: coveragePct,
-                })
-            : t('layers.traffic.simulatedNoMatches'),
+              ? t(
+                  'LIVE · Roads: OpenStreetMap · Flow: TomTom · {percent}% cov · Unmatched: simulated',
+                  {
+                    percent: coveragePct,
+                  },
+                )
+              : t(
+                  'LIVE · Roads: OpenStreetMap · Flow: TomTom · {percent}% cov',
+                  {
+                    percent: coveragePct,
+                  },
+                )
+            : t('SIMULATED · Roads: OpenStreetMap · Flow: TomTom (no matches)'),
       };
     }
     // Keyless simulation — one terse line that names the mode and the remedy
@@ -312,8 +320,8 @@ export function createModel({ state: layerState, services, parts, source }) {
       mode,
       error: null,
       loadingLabel: statusUnavailable
-        ? t('layers.traffic.simUnavailable')
-        : t('layers.traffic.simKeyless'),
+        ? t('SIMULATED — traffic service unreachable')
+        : t('SIMULATED — add TomTom key for live'),
     };
   }