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/Dockerfile

@@ -2,7 +2,7 @@
 # a golang-alpine builder yields a static binary shipped on distroless nonroot.
 #
 # The dictionary DAWGs are baked in from the scrabble-dictionary release artifact
-# (Stage 14) — the same set the Go CI downloads — and BACKEND_DICT_DIR points the
+# — the same set the Go CI downloads — and BACKEND_DICT_DIR points the
 # binary at them. The published solver module is fetched directly from Gitea
 # (GOPRIVATE), so the build stage needs git and network.
 #