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/src/Landing.svelte

@@ -6,7 +6,7 @@
   import { aboutContent } from './lib/aboutContent';
   import { telegramChannelLink } from './lib/landing';
 
-  // Standalone landing page (Stage 17), the public entry at "/" (the game SPA lives at /app/ and
+  // Standalone landing page, the public entry at "/" (the game SPA lives at /app/ and
   // /telegram/). It reuses the app's theme/i18n/prefs leaf modules — not the app store — so it
   // stays light. Theme is EPHEMERAL here (no auto, no persistence): it starts from the system
   // scheme and the icon toggles light<->dark in memory. Language IS persisted (synced with the app).
@@ -192,7 +192,7 @@
     font-size: 1.05rem;
   }
   /* The Telegram entry is just the bigger logo (no button chrome, no caption); the link
-     keeps an aria-label for assistive tech (Stage 17). */
+     keeps an aria-label for assistive tech. */
   .tg {
     align-self: center;
     display: inline-flex;