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/internal/server/server.go

@@ -50,20 +50,20 @@ type Deps struct {
 	// func skips the session-readiness check.
 	SessionsReady func() bool
 	// Sessions, Accounts and Games are the identity, account and game-domain
-	// services the Stage 6 REST handlers route to.
+	// services the REST handlers route to.
 	Sessions *session.Service
 	Accounts *account.Store
 	Games    *game.Service
-	// Social, Matchmaker, Invitations and Emails are the Stage 4 domain services
-	// the Stage 6 REST handlers route to.
+	// Social, Matchmaker, Invitations and Emails are the domain services
+	// the REST handlers route to.
 	Social      *social.Service
 	Matchmaker  *lobby.Matchmaker
 	Invitations *lobby.InvitationService
 	Emails      *account.EmailService
-	// Links drives account linking & merge (Stage 11): the /api/v1/user/link
+	// Links drives account linking & merge: the /api/v1/user/link
 	// endpoints. A nil Links disables them.
 	Links *link.Service
-	// Registry holds the resident dictionaries; the admin console (Stage 10) reads
+	// Registry holds the resident dictionaries; the admin console reads
 	// its versions and hot-reloads new ones. DictDir is the dictionary directory a
 	// reload reads a version subdirectory from. A nil Registry disables the console.
 	Registry *engine.Registry
@@ -72,7 +72,7 @@ type Deps struct {
 	// nil when BACKEND_CONNECTOR_ADDR is unset (broadcasts show a "not configured"
 	// notice).
 	Connector *connector.Client
-	// RateWatch ingests the gateway's rate-limiter rejection reports (R3): the
+	// RateWatch ingests the gateway's rate-limiter rejection reports: the
 	// admin console's throttled view + the high-rate auto-flag. A nil RateWatch
 	// disables the internal report endpoint and the console view.
 	RateWatch *ratewatch.Watch
@@ -196,16 +196,16 @@ func (s *Server) UserGroup() *gin.RouterGroup { return s.user }
 // InternalGroup returns the gateway-facing internal route group.
 func (s *Server) InternalGroup() *gin.RouterGroup { return s.internal }
 
-// Social returns the social domain service for the handlers added in Stage 6.
+// Social returns the social domain service for the handlers.
 func (s *Server) Social() *social.Service { return s.social }
 
-// Matchmaker returns the in-memory matchmaking pool for the Stage 6 handlers.
+// Matchmaker returns the in-memory matchmaking pool for the handlers.
 func (s *Server) Matchmaker() *lobby.Matchmaker { return s.matchmaker }
 
-// Invitations returns the friend-game invitation service for the Stage 6 handlers.
+// Invitations returns the friend-game invitation service for the handlers.
 func (s *Server) Invitations() *lobby.InvitationService { return s.invitations }
 
-// Emails returns the email confirm-code service for the Stage 6 handlers.
+// Emails returns the email confirm-code service for the handlers.
 func (s *Server) Emails() *account.EmailService { return s.emails }
 
 // Handler returns the underlying HTTP handler. It lets tests drive the server