Implement priority token.

internal/game/game_test.go

@@ -393,12 +393,17 @@ func TestFullGameFlow(t *testing.T) {
 	g, p1, p2 := testGame(t)
 	p1.Deck = append(p1.Deck, g.pet("Champ", 9))
 	p2.Deck = append(p2.Deck, g.pet("Chump", 1))
+	// testGame pins the priority token to seat 0, and seat 0 wins every
+	// battle, so the token moves to seat 1 after round 1 and stays there (a
+	// loser keeps it). Whoever holds it shops first.
+	wantPriority := p1.Seat
 	for round := 1; round <= MaxRounds; round++ {
 		if g.Round != round || g.Phase != PhaseShop {
 			t.Fatalf("expected shop of round %d, got round %d phase %s", round, g.Round, g.Phase)
 		}
-		if g.Turn != (round-1)%len(g.Players) {
-			t.Fatalf("round %d should rotate the starting player, turn=%d", round, g.Turn)
+		if g.PrioritySeat != wantPriority || g.Turn != wantPriority {
+			t.Fatalf("round %d: priority holder should shop first (want seat %d, priority=%d turn=%d)",
+				round, wantPriority, g.PrioritySeat, g.Turn)
 		}
 		for _, c := range g.ShopRow {
 			if c.ID != "" && c.Tier != round {
@@ -417,6 +422,10 @@ func TestFullGameFlow(t *testing.T) {
 		if g.Battle.WinnerSeat != p1.Seat {
 			t.Fatalf("round %d: seat 0 should win", round)
 		}
+		// The winner hands the token to the loser; a loser keeps it.
+		if wantPriority == g.Battle.WinnerSeat {
+			wantPriority = (wantPriority + 1) % len(g.Players)
+		}
 		for _, p := range g.Players {
 			if err := g.AcknowledgeBattle(p.ID); err != nil {
 				t.Fatal(err)