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/social/social.go

@@ -32,7 +32,7 @@ type GameReader interface {
 	// has moved); the nudge cooldown resets once the player has taken a turn.
 	LastMoveAt(ctx context.Context, gameID, accountID uuid.UUID) (time.Time, bool, error)
 	// GameLanguage is the game's language tag ("en"/"ru"), so a nudge's out-of-app push routes
-	// to the game's bot rather than the recipient's last-login bot (Stage 17).
+	// to the game's bot rather than the recipient's last-login bot.
 	GameLanguage(ctx context.Context, gameID uuid.UUID) (string, error)
 }
 
@@ -75,7 +75,7 @@ var (
 	ErrGameNotActive = errors.New("social: game is not active")
 	// ErrChatNotYourTurn is returned when a chat message is sent while it is not the
 	// sender's turn — chat is allowed only on your own turn (the opponent's-turn control
-	// is the nudge, Stage 17).
+	// is the nudge).
 	ErrChatNotYourTurn = errors.New("social: cannot chat while it is not your turn")
 )