Persist painted maps and HUD layout across sessions

Add MeshPainter.Persistence.cs: auto-save the three canonical paint maps
(albedo, MSO, emission) as PNGs under persistentDataPath on quit/play-stop
and auto-load them as the backend seed on start, so painting survives
sessions. Reuses the existing ReadCanonical/SetupBackend seed round-trip;
no new color-space code.

Add PaintHud.Persistence.cs: auto-save draggable panel positions and the
widget-bound painter values to a small JSON layout file, restored on start
via the painter -> SyncFromPainter -> widgets path. Complete SyncFromPainter
to also cover the spray sliders.

Assets/Scripts/PaintHud.cs

@@ -25,7 +25,7 @@ using UnityEngine.InputSystem.UI;
 /// sliders, an HSV picker + intensity for emission, and brush size. Attach to an empty GameObject.
 /// </summary>
 [DisallowMultipleComponent]
-public class PaintHud : MonoBehaviour
+public partial class PaintHud : MonoBehaviour
 {
     [Tooltip("Mode manager driving visibility. Empty = found automatically.")]
     public GameModeManager modeManager;
@@ -115,6 +115,9 @@ public class PaintHud : MonoBehaviour
         // Re-sync every widget when the pipette adopts a sampled material (painter -> HUD).
         if (painter) painter.Picked += SyncFromPainter;
 
+        // Restore the saved panel positions + widget values from the previous session.
+        RestoreHudState();
+
         // Set initial visibility (the manager's first SwitchTo may have fired before this Start).
         OnModeChanged(modeManager ? modeManager.ActiveMode : null);
     }
@@ -122,6 +125,7 @@ public class PaintHud : MonoBehaviour
     private void OnDestroy()
     {
         if (painter) painter.Picked -= SyncFromPainter;
+        SaveHudOnTeardown();
     }
 
     private void OnModeChanged(GameMode mode)
@@ -240,6 +244,12 @@ public class PaintHud : MonoBehaviour
         SetSlider(panel, SliderOcclusionAlpha, painter.occlusionOpacity);
         SetSlider(panel, SliderEmissionIntensity, painter.emissionIntensity);
         SetSlider(panel, SliderBrushSize, painter.brushSize);
+
+        SetSlider(panel, SliderSprayDots, painter.sprayDots);
+        SetSlider(panel, SliderSprayDotSizeMin, painter.sprayDotSizeMin);
+        SetSlider(panel, SliderSprayDotSizeMax, painter.sprayDotSizeMax);
+        SetSlider(panel, SliderSprayDotAlphaMin, painter.sprayDotAlphaMin);
+        SetSlider(panel, SliderSprayDotAlphaMax, painter.sprayDotAlphaMax);
     }
 
     private static void SetToggle(Transform panel, string name, bool value)