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))