feat: Introduce DownCount feature with new models, scripts, textures, and a dedicated quiz scene.

Assets/Quiz/Scripts/DownCount/LetterCard.cs

@@ -0,0 +1,46 @@
+
+using UdonSharp;
+using UnityEngine;
+using VRC.SDKBase;
+using VRC.Udon;
+
+public class LetterCard : UdonSharpBehaviour
+{
+    public TMPro.TextMeshProUGUI letterText;
+    [UdonSynced] public string currentLetter;
+
+    [HideInInspector] public LetterPoint currentPoint;
+
+    public override void OnDrop()
+    {
+        if (currentPoint != null)
+        {
+            currentPoint.OnCardReleased(this);
+        }
+    }
+
+    public override void OnPickup()
+    {
+        if (currentPoint != null)
+        {
+            currentPoint.OnCardRemoved(this);
+        }
+    }
+
+    public void SetLetter(string letter)
+    {
+        this.currentLetter = letter;
+        if (letterText != null)
+        {
+            letterText.text = currentLetter;
+        }
+    }
+
+    public override void OnDeserialization()
+    {
+        if (letterText != null)
+        {
+            letterText.text = currentLetter;
+        }
+    }
+}