Added isDirty

CharacterStat.cs

// Add these variables
private bool isDirty = true;
private float _value;

// Change the Value property to this
public float Value {
    get {
        if(isDirty) {
            _value = CalculateFinalValue();
            isDirty = false;
        }
        return _value;
    }
}

// Change the AddModifier method
public void AddModifier(StatModifier mod)
{
    isDirty = true;
    statModifiers.Add(mod);
}

// And change the RemoveModifier method
public bool RemoveModifier(StatModifier mod)
{
    isDirty = true;
    return statModifiers.Remove(mod);
}