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.

gateway/internal/backendclient/client.go

@@ -129,7 +129,7 @@ func (c *Client) SubscribePush(ctx context.Context, gatewayID string) (grpc.Serv
 // ReportRateLimited posts the gateway's periodic rate-limiter rejection summary
 // to the backend, which feeds the admin console's throttled view and the
 // high-rate auto-flag. The endpoint carries no user identity: like
-// sessions/resolve it rides the trusted internal segment (R3).
+// sessions/resolve it rides the trusted internal segment.
 func (c *Client) ReportRateLimited(ctx context.Context, windowSeconds int, entries []ratelimit.Rejection) error {
 	body := struct {
 		WindowSeconds int                   `json:"window_seconds"`