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/engine/alphabet_test.go

@@ -8,7 +8,7 @@ import (
 
 // TestAlphabetTableEnglish pins the English table against the solver ruleset: 26 letters,
 // contiguous indices, the concrete lower-case characters the solver emits and the standard
-// tile values. This is the real parity check the UI no longer carries (Stage 13).
+// tile values. This is the real parity check the UI no longer carries.
 func TestAlphabetTableEnglish(t *testing.T) {
 	tab, err := AlphabetTable(VariantEnglish)
 	if err != nil {