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/inttest/account_test.go

@@ -11,6 +11,8 @@ import (
 	"github.com/google/uuid"
 
 	"scrabble/backend/internal/account"
+	"scrabble/backend/internal/engine"
+	"scrabble/backend/internal/game"
 )
 
 // TestAccountProvisionByIdentity covers find-or-create semantics, distinct
@@ -78,7 +80,7 @@ func TestAccountProvisionByIdentity(t *testing.T) {
 }
 
 // TestGetStatsZeroForFreshAccount checks that an account with no finished games
-// reads back the zero statistics rather than an error (the Stage 8 stats screen).
+// reads back the zero statistics rather than an error (the stats screen).
 func TestGetStatsZeroForFreshAccount(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
@@ -109,7 +111,7 @@ func identityConfirmed(t *testing.T, kind, externalID string) bool {
 // TestProvisionTelegramSeedsNewAccountOnly checks that Telegram first contact
 // seeds the new account's language and display name from the launch fields,
 // defaults the in-app-only flag on, and never overwrites an existing account on a
-// later login (Stage 9 language seeding).
+// later login (language seeding).
 func TestProvisionTelegramSeedsNewAccountOnly(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
@@ -196,7 +198,7 @@ func TestServiceLanguageRoundTrip(t *testing.T) {
 	}
 }
 
-// TestHighRateFlagRoundTrip covers the R3 soft high-rate marker: a fresh account
+// TestHighRateFlagRoundTrip covers the soft high-rate marker: a fresh account
 // is unflagged, FlagHighRate stamps it exactly once (a second sustained episode
 // never moves the timestamp), ClearHighRateFlag reverses it, and a re-flag after
 // the operator clear takes a fresh timestamp.
@@ -279,7 +281,7 @@ func TestIdentityExternalID(t *testing.T) {
 	}
 }
 
-// TestNotificationsInAppOnlyRoundTrip checks the Stage 9 profile flag persists
+// TestNotificationsInAppOnlyRoundTrip checks the profile flag persists
 // through UpdateProfile and reads back through GetByID.
 func TestNotificationsInAppOnlyRoundTrip(t *testing.T) {
 	ctx := context.Background()
@@ -311,3 +313,72 @@ func TestNotificationsInAppOnlyRoundTrip(t *testing.T) {
 		t.Error("GetByID still reports in-app-only after clearing")
 	}
 }
+
+// provisionGuest creates a fresh ephemeral guest account and returns its id.
+func provisionGuest(t *testing.T) uuid.UUID {
+	t.Helper()
+	acc, err := account.NewStore(testDB).ProvisionGuest(context.Background())
+	if err != nil {
+		t.Fatalf("provision guest: %v", err)
+	}
+	if !acc.IsGuest {
+		t.Fatalf("provisioned account %s is not flagged guest", acc.ID)
+	}
+	return acc.ID
+}
+
+// TestGuestAutoMatchLeavesNoStats drives a guest through a full auto-match game
+// against a robot to a natural end and checks the guest holds a seat (the
+// game_players foreign key is satisfied) yet accrues no statistics, while the
+// durable robot opponent does.
+func TestGuestAutoMatchLeavesNoStats(t *testing.T) {
+	ctx := context.Background()
+	svc := newGameService()
+	robots := newRobotService(t, svc)
+	if err := robots.EnsurePool(ctx); err != nil {
+		t.Fatalf("ensure pool: %v", err)
+	}
+	robotID, err := robots.Pick(engine.VariantEnglish)
+	if err != nil {
+		t.Fatalf("pick: %v", err)
+	}
+	guest := provisionGuest(t)
+	seed := openingSeed(t)
+
+	g, err := svc.Create(ctx, game.CreateParams{
+		Variant: engine.VariantEnglish, Seats: []uuid.UUID{guest, robotID},
+		TurnTimeout: 24 * time.Hour, Seed: seed,
+	})
+	if err != nil {
+		t.Fatalf("create: %v", err)
+	}
+	const robotSeat = 1 // seats = [guest, robot]
+
+	finished := false
+	for i := 0; i < 400 && !finished; i++ {
+		_, toMove, status, err := svc.Participants(ctx, g.ID)
+		if err != nil {
+			t.Fatalf("participants: %v", err)
+		}
+		if status != game.StatusActive {
+			finished = true
+			break
+		}
+		if toMove == robotSeat {
+			setTurnStarted(t, g.ID, daytime.Add(-2*time.Hour))
+			robots.Drive(ctx, daytime)
+			continue
+		}
+		playHuman(t, ctx, svc, g.ID, guest)
+	}
+	if !finished {
+		t.Fatal("guest game did not finish within the move budget")
+	}
+
+	if _, _, _, _, _, ok := readStats(t, guest); ok {
+		t.Error("a guest must not accrue a statistics row")
+	}
+	if _, _, _, _, _, ok := readStats(t, robotID); !ok {
+		t.Error("the durable robot opponent should have a statistics row")
+	}
+}