Add tower Sell action with refund + coin VFX/SFX

Owner-validated sell RPC refunds SellRefundPercent of invested gold (Wall refunds 100% while un-upgraded), despawns the tower, and broadcasts a coin-burst VFX + rustle SFX. Refunds do not count as per-wave income. Investment/upgrade tracking added as the seam for the future upgrade system.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/_Project/Scripts/VFX/SellEffectSpawner.cs

@@ -0,0 +1,83 @@
+// Assets/_Project/Scripts/VFX/SellEffectSpawner.cs
+using UnityEngine;
+using TD.Audio;
+
+namespace TD.VFX
+{
+    /// <summary>
+    /// Scene singleton that plays the "tower sold" feedback — a burst of gold coins plus a
+    /// coin-rustle sound — at a world position. Mirrors <see cref="TD.UI.FloatingTextSpawner"/>:
+    /// visual-only, plain <c>MonoBehaviour</c>, invoked on every peer via a ClientRpc so all
+    /// players see and hear a sale locally.
+    /// </summary>
+    /// <remarks>
+    /// <b>Who calls this:</b> <see cref="TD.Gameplay.TowerPlacementManager.BroadcastSellEffect"/>
+    /// routes a ClientRpc here when a tower is sold.
+    ///
+    /// <b>Inspector setup:</b> drop this on a <c>SellEffectSpawner</c> GameObject in each Match
+    /// scene. The VFX prefab is OPTIONAL — leave it empty to use the built-in
+    /// <see cref="CoinBurstVfx"/> placeholder, or assign an authored particle/VFX-graph prefab.
+    /// Assign the coin-rustle clip on <see cref="sellSound"/>.
+    /// </remarks>
+    public class SellEffectSpawner : MonoBehaviour
+    {
+        // ----- Singleton --------------------------------------------------
+
+        public static SellEffectSpawner Instance { get; private set; }
+
+        // ----- Inspector --------------------------------------------------
+
+        [Tooltip("Optional authored VFX prefab spawned at the sale position. Leave empty to " +
+                 "fall back to the built-in code-generated CoinBurstVfx placeholder.")]
+        [SerializeField] private GameObject coinBurstPrefab;
+
+        [Tooltip("Coin-rustle sound played (2D, via AudioManager) when a tower is sold.")]
+        [SerializeField] private SoundConfig sellSound;
+
+        [Tooltip("Vertical offset above the sale position where the coin burst originates, so " +
+                 "it reads as coming from the tower body rather than the ground.")]
+        [SerializeField] private float verticalOffset = 0.75f;
+
+        // ----- Lifecycle --------------------------------------------------
+
+        private void Awake()
+        {
+            if (Instance != null && Instance != this)
+            {
+                Debug.LogWarning("[SellEffectSpawner] Duplicate instance detected. " +
+                                 "Only one should exist per scene.");
+                return;
+            }
+            Instance = this;
+        }
+
+        private void OnDestroy()
+        {
+            if (Instance == this) Instance = null;
+        }
+
+        // ----- Public API -------------------------------------------------
+
+        /// <summary>Plays the coin burst + rustle sound at <paramref name="worldPos"/>.</summary>
+        public void Play(Vector3 worldPos)
+        {
+            Vector3 spawnPos = worldPos + Vector3.up * verticalOffset;
+
+            if (coinBurstPrefab != null)
+            {
+                Instantiate(coinBurstPrefab, spawnPos, Quaternion.identity);
+            }
+            else
+            {
+                // Zero-art fallback: a self-configuring, self-destroying particle burst.
+                var go = new GameObject("CoinBurst");
+                go.transform.position = spawnPos;
+                go.AddComponent<CoinBurstVfx>();
+            }
+
+            if (sellSound.clip != null)
+                AudioManager.Instance?.Play(sellSound.clip, AudioCategory.UI,
+                                            sellSound.RandomPitch(), sellSound.volume);
+        }
+    }
+}