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.

backend/internal/ratewatch/ratewatch.go

@@ -1,5 +1,5 @@
 // Package ratewatch ingests the gateway's periodic rate-limiter rejection
-// reports (R3). It keeps an in-memory window of recent throttle episodes for
+// reports. It keeps an in-memory window of recent throttle episodes for
 // the admin console's view and applies the conservative high-rate auto-flag:
 // when one account's rejections within the rolling window cross the threshold,
 // the account store stamps the soft, reversible flagged_high_rate_at marker