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.

README.md

@@ -8,14 +8,13 @@ supports English Scrabble, Russian Scrabble and Эрудит.
 
 - **`gateway`** — the only public ingress: anti-abuse, platform authentication
   (resolves the player and injects `X-User-ID`), routing to `backend`, and an
-  admin surface behind Basic Auth. *(added in a later stage)*
+  admin surface behind Basic Auth.
 - **`backend`** — internal-only service that owns every domain concern and
   embeds the [`scrabble-solver`](../scrabble-solver) engine library in-process.
 - **`ui`** — pure-HTML5 client (plain Svelte 5 + TypeScript + Vite) over Connect-RPC
   + FlatBuffers, embeddable in platform webviews and packageable to native via
   Capacitor. See [`ui/README.md`](ui/README.md).
 - **`platform/*`** — per-platform side-services (e.g. the Telegram bot).
-  *(added in a later stage)*
 
 ## Documentation (sources of truth)
 
@@ -98,6 +97,6 @@ docker compose -f deploy/docker-compose.yml config         # validate (needs the
 
 CI auto-deploys the **test contour** on a PR into — or push to — `development`
 (`.gitea/workflows/ci.yaml`); the **prod contour** is a manual deploy after
-`development → master` (Stage 18). Env reference: [`deploy/.env.example`](deploy/.env.example);
+`development → master`. Env reference: [`deploy/.env.example`](deploy/.env.example);
 the topology and the two-contour model are in
 [`docs/ARCHITECTURE.md`](docs/ARCHITECTURE.md) §13.