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/connectsrv/peerip_test.go

@@ -7,7 +7,7 @@ import (
 
 // TestPeerIP covers the client-IP extraction the chat-moderation IP and the per-IP rate
 // limiter both rely on: the first X-Forwarded-For hop (the real client, once Caddy is
-// configured to trust its upstream), falling back to the connection peer (Stage 17).
+// configured to trust its upstream), falling back to the connection peer.
 func TestPeerIP(t *testing.T) {
 	tests := []struct {
 		name string