Fix player reconnection by re-registering sync context

When an existing player reconnects to a game, their sync context
is now updated so they continue receiving real-time updates.

game/store.go

@@ -196,6 +196,13 @@ func (gi *GameInstance) GetPlayerColor(pid PlayerID) int {
 	return 0
 }
 
+func (gi *GameInstance) RegisterSync(playerID PlayerID, sync Syncable) {
+	gi.playersMu.Lock()
+	gi.players[playerID] = sync
+	gi.playersMu.Unlock()
+	gi.dirty = true
+}
+
 func (gi *GameInstance) CreateRematch(gs *GameStore) *GameInstance {
 	gi.gameMu.Lock()
 	defer gi.gameMu.Unlock()