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/notify/notify_test.go

@@ -109,12 +109,12 @@ func TestOpponentMovedPayloadRoundTrips(t *testing.T) {
 		t.Fatalf("kind = %q", in.Kind)
 	}
 	ev := fb.GetRootAsOpponentMovedEvent(in.Payload, 0)
-	// The pre-R4 summary scalars repeat the move.
+	// The summary scalars repeat the move.
 	if string(ev.GameId()) != gid.String() || ev.Seat() != 1 || string(ev.Action()) != "play" || ev.Score() != 24 || ev.Total() != 130 {
 		t.Fatalf("scalars wrong: game=%q seat=%d action=%q score=%d total=%d",
 			ev.GameId(), ev.Seat(), ev.Action(), ev.Score(), ev.Total())
 	}
-	// The R4 delta: the move, the post-move summary and the bag size.
+	// The delta: the move, the post-move summary and the bag size.
 	if ev.BagLen() != 42 {
 		t.Fatalf("bag_len = %d, want 42", ev.BagLen())
 	}