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.

gateway/README.md

@@ -23,7 +23,7 @@ proto/edge/v1/          # Connect envelope contract (committed generated Go)
 internal/config/        # GATEWAY_* env config
 internal/backendclient/ # typed REST client (+ X-User-ID) and push gRPC client
 internal/session/       # in-memory session cache (LRU/TTL, backend fallback)
-internal/ratelimit/     # token-bucket limiter (golang.org/x/time/rate) + the rejection tracker (R3)
+internal/ratelimit/     # token-bucket limiter (golang.org/x/time/rate) + the rejection tracker
 internal/connector/     # gRPC client to the Telegram connector (initData validate, out-of-app push) + routing
 internal/push/          # live-event fan-out hub (per-user client streams)
 internal/transcode/     # FlatBuffers<->REST bridge + message_type registry
@@ -50,21 +50,21 @@ failures become Connect error codes.
 out-of-app push to that connector for recipients with no live in-app stream
 (ARCHITECTURE.md §10). When `GATEWAY_CONNECTOR_ADDR` is unset, both are disabled.
 
-The Stage 6 message-type slice: `auth.telegram`, `auth.guest`,
+The message-type catalog: `auth.telegram`, `auth.guest`,
 `auth.email.request`, `auth.email.login`, `profile.get`, `game.submit_play`,
-`game.state`, `lobby.enqueue`, `lobby.poll`, `chat.post`; live events
-`your_turn`, `opponent_moved`, `chat_message`, `nudge`, `match_found` (R4 enriched the game events —
-and `game_over`/`notify` — to carry the state delta the client applies without a `game.state`
-refetch). Stage 7
-added the play-loop ops; **Stage 8** added the social/account/history ops —
+`game.state`, `lobby.enqueue`, `lobby.poll`, `chat.post` and the play-loop ops;
+live events
+`your_turn`, `opponent_moved`, `chat_message`, `nudge`, `match_found` (the game events —
+and `game_over`/`notify` — carry the state delta the client applies without a `game.state`
+refetch). The social/account/history ops —
 `friends.*` (list/incoming/request/respond/cancel/unfriend/code.issue/code.redeem),
 `blocks.*`, `invitation.*` (list/create/accept/decline/cancel), `profile.update`,
-`stats.get`, `game.gcg`, and the `notify` live event — all via the identical
-transcode pattern (`transcode_social.go`). **Stage 11** added account linking & merge
+`stats.get`, `game.gcg`, and the `notify` live event — go through the identical
+transcode pattern (`transcode_social.go`). Account linking & merge
 — `link.email.request/confirm/merge` and `link.telegram.confirm/merge`
 (`transcode_link.go`); the telegram ops validate the **Login Widget** payload via the
 connector (`ValidateLoginWidget`) and forward the trusted `external_id`. These
-**superseded** the Stage 8 `email.bind.*` ops, which were removed.
+**superseded** the former `email.bind.*` ops, which were removed.
 
 ## Configuration
 
@@ -81,13 +81,13 @@ connector (`ValidateLoginWidget`) and forward the trusted `external_id`. These
 | `GATEWAY_SESSION_TTL` | `10m` | cached session lifetime |
 | `GATEWAY_SESSION_CACHE_MAX` | `50000` | cached session cap |
 | `GATEWAY_PUSH_HEARTBEAT_INTERVAL` | `10s` | live-stream keep-alive (an immediate heartbeat also fires on open, under the ~15s edge idle timeout) |
-| `GATEWAY_MAX_BODY_BYTES` | `1048576` | caps one request body and one Connect message read; an oversized Execute is refused with `resource_exhausted` (R3) |
+| `GATEWAY_MAX_BODY_BYTES` | `1048576` | caps one request body and one Connect message read; an oversized Execute is refused with `resource_exhausted` |
 | `GATEWAY_SERVICE_NAME` | `scrabble-gateway` | OpenTelemetry `service.name` |
 | `GATEWAY_OTEL_TRACES_EXPORTER` | `none` | `none`, `stdout` or `otlp` (gRPC; endpoint from `OTEL_EXPORTER_OTLP_*`) |
 | `GATEWAY_OTEL_METRICS_EXPORTER` | `none` | `none`, `stdout` or `otlp` |
 
 Rate-limit defaults (built-in): public 30/min·IP (burst 10), authenticated
-300/min·user (burst 80, raised in Stage 17 for multi-device play), admin
+300/min·user (burst 80, sized for multi-device play), admin
 60/min·IP (burst 20, guarding the `/_gm` mount ahead of its Basic-Auth),
 email-code 5/10 min·IP (burst 2).
 
@@ -95,7 +95,7 @@ Every rejection increments `gateway_rate_limited_total{class}`
 (`user`/`public`/`email`/`admin`) and logs one Debug line; a reporter drains the
 per-key rejection tracker every 30 s, emits a Warn summary per throttled key and
 posts the report to the backend (`/api/v1/internal/ratelimit/report`), feeding
-the admin console's throttled view and the high-rate auto-flag (R3).
+the admin console's throttled view and the high-rate auto-flag.
 
 ## Run