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/session/service.go

@@ -73,8 +73,8 @@ func (svc *Service) Revoke(ctx context.Context, token string) error {
 }
 
 // RevokeAllForAccount revokes every active session of accountID and evicts them
-// from the cache. The account-merge flow calls it to retire a secondary account
-// (Stage 11). It is idempotent.
+// from the cache. The account-merge flow calls it to retire a secondary account.
+// It is idempotent.
 func (svc *Service) RevokeAllForAccount(ctx context.Context, accountID uuid.UUID) error {
 	if _, err := svc.store.RevokeAllForAccount(ctx, accountID, time.Now().UTC()); err != nil {
 		return err