[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/MultiplierForSixes.cs

@@ -0,0 +1,16 @@
+using UnityEngine;
+
+[CreateAssetMenu(fileName = "MultiplierForSixes", menuName = "YachtDice/Modifiers/Multiplier For Sixes")]
+public sealed class MultiplierForSixes : ScoreModifier
+{
+    [SerializeField] private float multiplierPerSix = 6f;
+
+    public override ModifierPhase Phase => ModifierPhase.Multiplicative;
+
+    public override void Apply(ref ScoreResult result)
+    {
+        for (int i = 0; i < result.DiceValues.Length; i++)
+            if (result.DiceValues[i] == 6)
+                result.Multiplier *= multiplierPerSix;
+    }
+}