[Refactor] Replace [SerializeField] + getter with [field: SerializeField] auto-properties

Convert serialized private fields with public arrow getters to
[field: SerializeField] auto-properties across ScriptableObjects
(DieDefinitionSO, CategoryDefinition, ModifierDefinition, Effect,
ModifierBehavior, SumOfValueCategory) and select MonoBehaviours
(DiceRoller, Dice.Entry struct). Fields with type-changing getters
(List→IReadOnlyList) or computed getters are intentionally kept as-is.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Assets/Scripts/Modifiers/Definition/ModifierBehavior.cs

@@ -8,11 +8,10 @@ namespace YachtDice.Modifiers.Definition
     [CreateAssetMenu(fileName = "NewBehavior", menuName = "YachtDice/Modifiers/Behavior")]
     public class ModifierBehavior : ScriptableObject
     {
-        [SerializeField] private TriggerType trigger;
+        [field: SerializeField] public TriggerType Trigger { get; private set; }
         [SerializeField] private List<Condition> conditions = new();
         [SerializeField] private List<Effect> effects = new();
 
-        public TriggerType Trigger => trigger;
         public IReadOnlyList<Condition> Conditions => conditions;
         public IReadOnlyList<Effect> Effects => effects;
 
@@ -33,7 +32,7 @@ namespace YachtDice.Modifiers.Definition
             List<Effect> effects)
         {
             var so = CreateInstance<ModifierBehavior>();
-            so.trigger = trigger;
+            so.Trigger = trigger;
             so.conditions = conditions ?? new List<Condition>();
             so.effects = effects ?? new List<Effect>();
             return so;