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/invitations.go

@@ -106,7 +106,7 @@ func (svc *InvitationService) SetNotifier(p notify.Publisher) {
 }
 
 // emitInvitation publishes the invitation notification to each invitee, carrying the invitation
-// itself so the client adds it to its lobby list without a refetch (R4).
+// itself so the client adds it to its lobby list without a refetch.
 func (svc *InvitationService) emitInvitation(ctx context.Context, inv Invitation, inviteeIDs []uuid.UUID) {
 	if len(inviteeIDs) == 0 {
 		return
@@ -120,7 +120,7 @@ func (svc *InvitationService) emitInvitation(ctx context.Context, inv Invitation
 }
 
 // emitGameStarted publishes the game_started notification to each seated player, carrying their
-// initial view of the started game so the client seeds its game cache without a refetch (R4). A
+// initial view of the started game so the client seeds its game cache without a refetch. A
 // seat whose state cannot be read is skipped (it still sees the game on the next lobby load).
 func (svc *InvitationService) emitGameStarted(ctx context.Context, g game.Game, seats []uuid.UUID) {
 	intents := make([]notify.Intent, 0, len(seats))

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)
 }
 

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)

backend/internal/lobby/matchmaker_test.go

@@ -249,7 +249,7 @@ func TestMatchmakerReaperSkipsCancelled(t *testing.T) {
 
 // TestMatchmakerCancelClearsPendingResult covers the race where the reaper substitutes a
 // robot just before the player cancels: Cancel must drop the pending result so the
-// abandoned game never surfaces through Poll (Stage 17).
+// abandoned game never surfaces through Poll.
 func TestMatchmakerCancelClearsPendingResult(t *testing.T) {
 	creator := &fakeCreator{}
 	mm := newTestMatchmaker(creator, uuid.New())