Implement priority token.

internal/game/game.go

@@ -76,18 +76,23 @@ type PendingTrade struct {
 // Game is the complete authoritative state. It is a pure state machine: no
 // goroutines, no clocks, no I/O. Callers are responsible for locking.
 type Game struct {
-	ID         string        `json:"id"`
-	Code       string        `json:"code"`
-	Phase      Phase         `json:"phase"`
-	Round      int           `json:"round"` // 1-based
-	Players    []*Player     `json:"players"`
-	ShopDecks  [][]Card      `json:"shopDecks"` // index 0 = tier 1
-	ShopRow    []Card        `json:"shopRow"`   // empty ID = empty slot
-	Turn       int           `json:"turn"`      // seat with the current shop turn
-	Pending    *PendingTrade `json:"pending,omitempty"`
-	Battle     *BattleResult `json:"battle,omitempty"` // most recent battle
-	NextCardID int           `json:"nextCardId"`
-	WinnerSeat int           `json:"winnerSeat"` // set at gameover; -1 = tie
+	ID        string    `json:"id"`
+	Code      string    `json:"code"`
+	Phase     Phase     `json:"phase"`
+	Round     int       `json:"round"` // 1-based
+	Players   []*Player `json:"players"`
+	ShopDecks [][]Card  `json:"shopDecks"` // index 0 = tier 1
+	ShopRow   []Card    `json:"shopRow"`   // empty ID = empty slot
+	Turn      int       `json:"turn"`      // seat with the current shop turn
+	// PrioritySeat holds the priority token: that seat shops first each round
+	// and wins simultaneity races in battle. Assigned randomly at game start;
+	// a battle winner hands it to the loser, a loser keeps it, a draw leaves
+	// it put.
+	PrioritySeat int           `json:"prioritySeat"`
+	Pending      *PendingTrade `json:"pending,omitempty"`
+	Battle       *BattleResult `json:"battle,omitempty"` // most recent battle
+	NextCardID   int           `json:"nextCardId"`
+	WinnerSeat   int           `json:"winnerSeat"` // set at gameover; -1 = tie
 
 	// RollDie overrides the rock die (faces 0,0,1,1,2,2) for tests. Nil
 	// (including after loading from storage) means a fair random roll.
@@ -199,6 +204,8 @@ func (g *Game) PlayerByID(id string) *Player {
 
 func (g *Game) start() {
 	g.Round = 1
+	// The priority token starts with a random seat.
+	g.PrioritySeat = randInt(len(g.Players))
 	g.startShopRound()
 }
 
@@ -216,7 +223,8 @@ func (g *Game) startShopRound() {
 	for i := range g.ShopRow {
 		g.ShopRow[i] = g.drawFromTier(g.Round)
 	}
-	g.Turn = (g.Round - 1) % len(g.Players)
+	// The priority-token holder shops first.
+	g.Turn = g.PrioritySeat
 }
 
 // drawFromTier pops the top card of the given tier's deck (1-based tier).