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/lobby/lobby.go

@@ -23,7 +23,7 @@ type GameCreator interface {
 	Create(ctx context.Context, params game.CreateParams) (game.Game, error)
 	// InitialState returns a seated player's full initial view of a started game, used
 	// to enrich the match_found / game_started events so the client renders the new game
-	// without a follow-up fetch (R4).
+	// without a follow-up fetch.
 	InitialState(ctx context.Context, gameID, accountID uuid.UUID) (notify.PlayerState, error)
 }