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/matchmaker.go

@@ -76,7 +76,7 @@ func (m *Matchmaker) SetNotifier(p notify.Publisher) {
 // emitMatchFound pushes match_found to every seat of a freshly started game.
 // Emitting to a robot seat is harmless (no client subscription exists for it).
 func (m *Matchmaker) emitMatchFound(ctx context.Context, g game.Game) {
-	lang := g.Variant.Language() // route the push by the game's language, not the recipient's bot (Stage 17)
+	lang := g.Variant.Language() // route the push by the game's language, not the recipient's bot
 	intents := make([]notify.Intent, 0, len(g.Seats))
 	for _, s := range g.Seats {
 		state, err := m.games.InitialState(ctx, g.ID, s.AccountID)