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/e2e/landing.spec.ts

@@ -1,7 +1,7 @@
 import { expect, test } from './fixtures';
 
 // The landing page is a separate Vite entry (landing.html), served at "/" in production while
-// the game SPA lives at /app/ and /telegram/ (Stage 17). In dev it is reachable at /landing.html.
+// the game SPA lives at /app/ and /telegram/. In dev it is reachable at /landing.html.
 test('landing shows the pitch, switches language via the dropdown, and toggles theme', async ({ page }) => {
   await page.goto('/landing.html');