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/internal/config/config.go

@@ -76,13 +76,13 @@ const (
 	defaultBackendTimeout        = 5 * time.Second
 	defaultSessionTTL            = 10 * time.Minute
 	defaultSessionCacheMax       = 50000
-	defaultPushHeartbeatInterval = 10 * time.Second // under the ~15 s edge idle timeout (Stage 17)
+	defaultPushHeartbeatInterval = 10 * time.Second // under the ~15 s edge idle timeout
 	defaultServiceName           = "scrabble-gateway"
 )
 
 // DefaultMaxBodyBytes is the default request-body cap (GATEWAY_MAX_BODY_BYTES):
 // 1 MiB — far above any legitimate edge payload (drafts and chat are a few KB)
-// yet small enough to stop a cheap memory-amplification upload (R3).
+// yet small enough to stop a cheap memory-amplification upload.
 const DefaultMaxBodyBytes = 1 << 20
 
 // supportedLanguages is the set of game languages a service may declare for the
@@ -98,8 +98,8 @@ func DefaultRateLimit() RateLimitConfig {
 		PublicPerMinute: 30, PublicBurst: 10,
 		// Per-user (not per-IP): one user may run several devices, each holding a
 		// Subscribe stream and reloading state on every live event, so the authenticated
-		// budget is generous (a per-user cap cannot DoS the service). Raised in Stage 17
-		// after multi-device play tripped the old 120/40.
+		// budget is generous (a per-user cap cannot DoS the service). It is raised
+		// because multi-device play tripped the old 120/40.
 		UserPerMinute: 300, UserBurst: 80,
 		AdminPerMinute: 60, AdminBurst: 20,
 		EmailPer10Min: 5, EmailBurst: 2,