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/cmd/backend/main.go

@@ -108,7 +108,7 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 		zap.String("dir", cfg.Game.DictDir),
 		zap.String("version", cfg.Game.DictVersion))
 
-	// Stage 10 admin console: an optional backend client to the Telegram connector
+	// Admin console: an optional backend client to the Telegram connector
 	// side-service for operator broadcasts. Unset (BACKEND_CONNECTOR_ADDR empty)
 	// leaves broadcasts disabled — the console shows a "not configured" notice.
 	var conn *connector.Client
@@ -141,7 +141,7 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	logger.Info("game turn-timeout sweeper started",
 		zap.Duration("interval", cfg.Game.TimeoutSweepInterval))
 
-	// Stage 12 TODO-3: reap abandoned guest accounts (no game seat, account age past
+	// Reap abandoned guest accounts (no game seat, account age past
 	// the retention window). Dependent rows fall away via ON DELETE CASCADE.
 	guestReaper := account.NewGuestReaper(accounts, cfg.GuestRetention, logger)
 	go guestReaper.Run(ctx, cfg.GuestReapInterval)
@@ -149,19 +149,18 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 		zap.Duration("interval", cfg.GuestReapInterval),
 		zap.Duration("retention", cfg.GuestRetention))
 
-	// Stage 4 lobby & social domains. Their REST and stream surface is added with
-	// the gateway in Stage 6, so they are handed to the server (like the route
-	// groups) for the handlers to come.
+	// Lobby & social domains. Their REST and stream surface lives in the gateway,
+	// so they are handed to the server (like the route groups) for the handlers.
 	mailer := newMailer(cfg.SMTP, logger)
 	emails := account.NewEmailService(accounts, mailer)
-	// Stage 11 account linking & merge: the orchestrator over the account, merge and
+	// Account linking & merge: the orchestrator over the account, merge and
 	// session layers. Wired to the /api/v1/user/link REST surface below.
 	links := link.NewService(emails, accounts, accountmerge.NewMerger(db), sessions)
 	socialSvc := social.NewService(social.NewStore(db), accounts, games)
 	socialSvc.SetNotifier(hub)
 	socialSvc.SetMetrics(tel.MeterProvider().Meter("scrabble/backend/social"))
 
-	// Stage 5 robot opponent: provision its durable account pool (a hard startup
+	// Robot opponent: provision its durable account pool (a hard startup
 	// dependency, like the dictionaries) and start its move driver. The matchmaker
 	// substitutes a pooled robot for a missing human after the wait window.
 	robots := robot.NewService(games, accounts, socialSvc, tel.MeterProvider().Meter("scrabble/backend/robot"), logger)
@@ -178,7 +177,7 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	invitations.SetNotifier(hub)
 	logger.Info("lobby and social domains ready", zap.Duration("robot_wait", cfg.Lobby.RobotWait))
 
-	// R3 rate-limit observability: ingest the gateway's rejection reports for the
+	// Rate-limit observability: ingest the gateway's rejection reports for the
 	// admin throttled view and the conservative high-rate auto-flag.
 	rateWatch := ratewatch.New(cfg.RateWatch, accounts, logger)
 	logger.Info("rate watch ready",