Persist HUD layout on drag-end instead of only on teardown

HUD panel positions and brush values were written only from
OnApplicationQuit/OnDestroy, which is not guaranteed in a build (Alt-F4,
process kill, crash), so layout work was lost. DraggablePanel now exposes
an IEndDragHandler callback that saves the layout the moment a panel is
dropped. Brush-value changes are detected against a disk baseline in
LateUpdate and flushed after a 0.5s debounce so per-frame slider drags
do not write every frame. The teardown save path is removed.

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

Assets/Scripts/DraggablePanel.cs

@@ -14,7 +14,7 @@ using UnityEngine.EventSystems;
 /// corners in its parent, not from a fixed top-left assumption).
 /// </summary>
 [RequireComponent(typeof(RectTransform))]
-public class DraggablePanel : MonoBehaviour, IPointerDownHandler, IBeginDragHandler, IDragHandler
+public class DraggablePanel : MonoBehaviour, IPointerDownHandler, IBeginDragHandler, IDragHandler, IEndDragHandler
 {
     [Tooltip("Panel moved by the drag. Empty = this object's RectTransform.")]
     public RectTransform target;
@@ -22,6 +22,10 @@ public class DraggablePanel : MonoBehaviour, IPointerDownHandler, IBeginDragHand
     [Tooltip("Panel pixels (canvas units) kept on screen on every edge so it can never be lost.")]
     public float keepVisible = 48f;
 
+    // Raised once when a real panel drag ends (not for gestures that began on a widget). The HUD uses
+    // this to persist the layout the moment a panel is dropped. Optional - left null by other users.
+    public System.Action Moved;
+
     private RectTransform _area; // the panel's parent; pointer positions are tracked in its local space
     private Vector2 _grabOffset;
     private bool _dragging;
@@ -66,6 +70,13 @@ public class DraggablePanel : MonoBehaviour, IPointerDownHandler, IBeginDragHand
         target.anchoredPosition = Clamp(local + _grabOffset);
     }
 
+    public void OnEndDrag(PointerEventData eventData)
+    {
+        if (!_dragging) return; // gesture began on a widget -> not our drag, nothing moved
+        _dragging = false;
+        Moved?.Invoke();
+    }
+
     // True when the press started on an interactive widget inside the panel (which then owns the gesture).
     private bool StartedOnWidget(PointerEventData eventData)
     {