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/cmd/gateway/main.go

@@ -42,10 +42,10 @@ const (
 	// readHeaderTimeout bounds reading one request's headers on the public
 	// listener (a slowloris guard). Bodies and long-lived streams are governed by
 	// the h2c settings in connectsrv — Read/WriteTimeout stay unset on purpose,
-	// they would kill the Subscribe stream (R3).
+	// they would kill the Subscribe stream.
 	readHeaderTimeout = 10 * time.Second
 	// throttleReportInterval is the cadence of the rate-limiter rejection
-	// summary: the Warn log per throttled key and the report to the backend (R3).
+	// summary: the Warn log per throttled key and the report to the backend.
 	throttleReportInterval = 30 * time.Second
 )
 
@@ -281,7 +281,7 @@ func deliverOutOfApp(ctx context.Context, backend *backendclient.Client, conn *c
 		return
 	}
 	// A game event carries its own language, so the push comes from the game's bot rather than
-	// the recipient's last-login bot (Stage 17); other events fall back to the service language.
+	// the recipient's last-login bot; other events fall back to the service language.
 	lang := target.Language
 	if gameLang != "" {
 		lang = gameLang