R6(a): de-stage code, docs, READMEs; split stage6_test

Mechanical, behaviour-preserving removal of Stage N / TODO-N / phase (RN)
references from comments, doc-comments, service READMEs, the current-state docs
(ARCHITECTURE, FUNCTIONAL+_ru, TESTING, UI_DESIGN), config-file comments, and the
.fbs/.proto schema comments. PLAN.md / PRERELEASE.md / CLAUDE.md keep the stage
history.

- Rename the only stage-named identifiers: registerStage8 -> registerSocialOps,
  registerStage11 -> registerLinkOps (gateway transcode).
- Split stage6_test.go: TestEmailLoginFlow -> email_test.go,
  TestGuestAutoMatchLeavesNoStats (+ provisionGuest) -> account_test.go.
- Regenerated proto bindings (push.pb.go, telegram_grpc.pb.go) from the de-staged
  .proto comments; FB Go/TS bindings unchanged (flatc strips schema comments).

go build/vet/gofmt clean across modules; integration typecheck and pnpm check green.

ui/vite.config.ts

@@ -15,7 +15,7 @@ export default defineConfig(({ mode }) => ({
   base: './',
   define: {
     // App version shown on the About screen, injected at build time from `git describe`
-    // via a Docker build-arg (Stage 17). Falls back to "dev" for a plain local/mock build,
+    // via a Docker build-arg. Falls back to "dev" for a plain local/mock build,
     // so a missing build-arg never breaks the build.
     __APP_VERSION__: JSON.stringify(process.env.VITE_APP_VERSION || 'dev'),
   },
@@ -35,7 +35,7 @@ export default defineConfig(({ mode }) => ({
   build: {
     target: 'es2022',
     sourcemap: true,
-    // Two entries (Stage 17): the game SPA (index.html, served at /app/ + /telegram/) and the
+    // Two entries: the game SPA (index.html, served at /app/ + /telegram/) and the
     // public landing page (landing.html, served at /). Assets are shared in dist/assets/, and
     // the relative base lets one build serve under any path.
     rollupOptions: {