fix(Bullets): 将箭矢的生命周期从距离改为时间

修改Arrow.tscn中的lifeDistance为lifeTime，并同步更新Arrow.gd中的相关计算逻辑，以统一使用时间作为生命周期基准

scripts/Contents/Bullets/Arrow.gd

@@ -18,7 +18,7 @@ func ai():
 	else:
 		trail.emitting = true
 		hitbox.disabled = false
-	speed = (1 - lifeDistancePercent()) * initialSpeed
+	speed = (1 - lifeTimePercent()) * initialSpeed
 	baseDamage = speed * atk
 	PresetBulletAI.forward(self, rotation)
 	if speed < 1: