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/PaintHud.cs

@@ -162,6 +162,7 @@ public partial class PaintHud : MonoBehaviour
         if (_hasCarry) ApplyCarry();
         else RestoreHudState();
         CaptureCarry(); // seed/refresh the mirror from the now-current painter
+        SnapshotSavedBaseline(); // values now match disk -> auto-save only fires on a real edit
 
         UpdateButtonStates(); // enable Save now that a painter is bound
     }
@@ -169,7 +170,6 @@ public partial class PaintHud : MonoBehaviour
     private void OnDestroy()
     {
         if (painter) painter.Picked -= SyncFromPainter;
-        SaveHudOnTeardown();
     }
 
     private void OnModeChanged(GameMode mode)
@@ -409,13 +409,14 @@ public partial class PaintHud : MonoBehaviour
     /// from any of its non-interactive surface (header, padding, labels, background); the component
     /// ignores presses that start on a widget (toggle, slider, colour picker).
     /// </summary>
-    private static void MakeDraggable(Transform root, string panelName)
+    private void MakeDraggable(Transform root, string panelName)
     {
         var panel = FindDeep(root, panelName) as RectTransform;
         if (!panel) return;
         var drag = panel.GetComponent<DraggablePanel>();
         if (!drag) drag = panel.gameObject.AddComponent<DraggablePanel>();
         drag.target = panel;
+        drag.Moved = OnPanelMoved; // persist the layout the moment a panel is dropped
     }
 
     private void EnsureFonts(Transform panel)