[Rename] Unify Die → Dice naming across the entire project

Standardize all class, interface, file, method, event, field, and variable
names from the inconsistent "Die" form to "Dice", matching the existing
DiceManager/DiceCatalog/DiceCollection convention.

Renamed files (7 + meta): IDie→IDice, DieInstance→DiceInstance,
DieDefinitionSO→DiceDefinitionSO, StandardDieSO→StandardDiceSO,
DieValueCondition→DiceValueCondition, AddPerDieEffect→AddPerDiceEffect,
MultiplyPerDieEffect→MultiplyPerDiceEffect.

Updated all 31 consumer and test files with matching reference changes.

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

Assets/Scripts/Shop/IShopItem.cs

@@ -4,7 +4,7 @@ namespace YachtDice.Shop
 {
     /// <summary>
     /// Any item that can appear in the shop.
-    /// Implemented by ScriptableObject definitions (ModifierDefinition, DieDefinitionSO).
+    /// Implemented by ScriptableObject definitions (ModifierDefinition, DiceDefinitionSO).
     /// </summary>
     public interface IShopItem
     {