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/game/service.go

@@ -217,14 +217,13 @@ func (svc *Service) Resign(ctx context.Context, gameID, accountID uuid.UUID) (Mo
 
 // GameVariant returns just a game's variant. The edge layer uses it to map wire alphabet
 // indices to concrete letters before delegating to the letter-based play, exchange and
-// word-check methods (Stage 13), keeping a single domain path shared with the robot.
+// word-check methods, keeping a single domain path shared with the robot.
 func (svc *Service) GameVariant(ctx context.Context, gameID uuid.UUID) (engine.Variant, error) {
 	return svc.store.GetGameVariant(ctx, gameID)
 }
 
 // GameLanguage returns the game's language tag ("en"/"ru"), derived from its variant, so a
-// game push routes out-of-app to the game's own bot rather than the recipient's last-login bot
-// (Stage 17).
+// game push routes out-of-app to the game's own bot rather than the recipient's last-login bot.
 func (svc *Service) GameLanguage(ctx context.Context, gameID uuid.UUID) (string, error) {
 	v, err := svc.GameVariant(ctx, gameID)
 	if err != nil {
@@ -241,7 +240,7 @@ func (svc *Service) RobotSchedule(ctx context.Context, gameID uuid.UUID) (seed i
 
 // LastMoveAt returns the time of an account's most recent move in a game (and whether it
 // has moved). The social service uses it to reset the nudge cooldown once a player has
-// taken a turn (Stage 17).
+// taken a turn.
 func (svc *Service) LastMoveAt(ctx context.Context, gameID, accountID uuid.UUID) (time.Time, bool, error) {
 	return svc.store.LastMoveAt(ctx, gameID, accountID)
 }
@@ -294,7 +293,7 @@ func (svc *Service) transition(ctx context.Context, gameID, accountID uuid.UUID,
 	return MoveResult{Move: rec, Game: post, Rack: g.Hand(seat), BagLen: g.BagLen()}, nil
 }
 
-// afterCommitDrafts maintains the Stage 17 drafts after a committed move: the actor's own
+// afterCommitDrafts maintains the drafts after a committed move: the actor's own
 // composition is consumed, so clear it; a play's tiles may overlap an opponent's board
 // draft, which is then reset. Best-effort — the move is already committed, so a draft
 // cleanup failure is logged rather than failing the move.
@@ -382,7 +381,7 @@ func (svc *Service) emitMove(ctx context.Context, post Game, rec engine.MoveReco
 		intents = append(intents, notify.OpponentMoved(s.AccountID, post.ID, rec, summary, bagLen))
 	}
 	// Game pushes are routed out-of-app by the game's own language, not the recipient's
-	// last-login bot (Stage 17).
+	// last-login bot.
 	lang := post.Variant.Language()
 	switch post.Status {
 	case StatusActive:
@@ -789,7 +788,7 @@ func (svc *Service) GameState(ctx context.Context, gameID, accountID uuid.UUID)
 }
 
 // InitialState returns accountID's full initial view of game gameID as the notify
-// PlayerState carried by the match_found / game_started events (R4), so a client can
+// PlayerState carried by the match_found / game_started events, so a client can
 // render a freshly started game from the event without a follow-up fetch. The variant
 // alphabet table is always embedded (the recipient may be seeing the variant for the
 // first time). It satisfies lobby.GameCreator.
@@ -837,7 +836,7 @@ func (svc *Service) ListForAccount(ctx context.Context, accountID uuid.UUID) ([]
 
 // HideGame hides a finished game from accountID's own lobby (it stays visible to the other
 // players); it is irreversible by design. Only a player of a finished game may hide it
-// (ErrNotAPlayer / ErrGameActive otherwise); hiding an already-hidden game is a no-op (Stage 17).
+// (ErrNotAPlayer / ErrGameActive otherwise); hiding an already-hidden game is a no-op.
 func (svc *Service) HideGame(ctx context.Context, accountID, gameID uuid.UUID) error {
 	g, err := svc.store.GetGame(ctx, gameID)
 	if err != nil {