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/social/friends.go

@@ -31,7 +31,7 @@ const friendRequestTTL = 30 * 24 * time.Hour
 
 // accountRef resolves accountID into a notify.AccountRef (the display name from the account
 // store, empty on a lookup failure), for enriching the friend_* live events so the client
-// updates its requests/friends state without a refetch (R4).
+// updates its requests/friends state without a refetch.
 func (svc *Service) accountRef(ctx context.Context, accountID uuid.UUID) notify.AccountRef {
 	ref := notify.AccountRef{AccountID: accountID.String()}
 	if acc, err := svc.accounts.GetByID(ctx, accountID); err == nil {