[Add] Yacht scoring system with Balatro-like modifier pipeline

Implements the core game loop for Yacht dice: 5-dice rolling with
lock/unlock, 3 rolls per turn, 13 standard scoring categories, and
an extensible ScriptableObject-based modifier system that applies
additive then multiplicative bonuses (chips+mult pattern).

Includes two test modifiers: BonusForOnes (+10 per 1) and
MultiplierForSixes (x6 per 6).

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

Assets/Scripts/Modifiers/ScoreModifier.cs

@@ -0,0 +1,19 @@
+using UnityEngine;
+
+public abstract class ScoreModifier : ScriptableObject
+{
+    public enum ModifierPhase
+    {
+        Additive,
+        Multiplicative
+    }
+
+    [SerializeField] private string displayName;
+    [SerializeField] [TextArea] private string description;
+
+    public string DisplayName => displayName;
+    public string Description => description;
+
+    public abstract ModifierPhase Phase { get; }
+    public abstract void Apply(ref ScoreResult result);
+}