[Unity] 본진 업그레이드 시스템

tables.Add(building.Skill?.UpgradeLevels ?? new List<BuildingAsset.SkillUpgradeLevel>());

생산 건물 3종:  [0, 0, 1, 1, 2, 2]   → 본진 한 단계당 2레벨
마법 연구소:    [0, 0, 0, 1, 2]      → 처음 3레벨, 이후 단계당 1레벨

public abstract class UpgradeStep
{
    public List<ResourceCost> Cost;    // 이 레벨 도달 비용
    public int RequiredCastleLevel;    // 열리는 데 필요한 본진 레벨 (0 = 처음부터)
}

public IReadOnlyList<UpgradeStep> UpgradeSteps
{
    get
    {
        if (HasSteps(Castle?.UpgradeLevels)) return Castle.UpgradeLevels;
        if (HasSteps(Skill?.UpgradeLevels)) return Skill.UpgradeLevels;
        if (HasSteps(Production?.UpgradeLevels)) return Production.UpgradeLevels;
        if (HasSteps(Exchange?.UpgradeLevels)) return Exchange.UpgradeLevels;
        return Array.Empty<UpgradeStep>();
    }
}

- tables.Add(building.Skill?.UpgradeLevels ?? new List<SkillUpgradeLevel>());
+ tables.Add(building.UpgradeSteps);

public int LineMaxLevel(int index) => _lineUpgradeLevels[index].Count;

private int EffectiveMaxLevel(IReadOnlyList<UpgradeStep> levels, bool ignoreGate = false)
{
    if (ignoreGate) return levels.Count;

    int castle = CastleLevel;
    for (int i = 0; i < levels.Count; i++)
    {
        if (levels[i] == null || levels[i].RequiredCastleLevel > castle) return i;
    }
    return levels.Count;
}