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/MeshPainter.cs

@@ -295,7 +295,9 @@ public partial class MeshPainter : MonoBehaviour
         else if (_materialInstance.HasProperty(ColorId)) _materialInstance.SetColor(ColorId, Color.white);
 
         BuildSeedColors();
-        SetupBackend(useGpu, _seedCanonical);
+        // Persistence: seed from the saved maps if present (per-map null falls back to the
+        // material seed), otherwise from the material as before.
+        SetupBackend(useGpu, LoadPaintMaps() ?? _seedCanonical);
         ConfigureMaterialPbr();
 
         EnsureProceduralMask();
@@ -841,8 +843,16 @@ public partial class MeshPainter : MonoBehaviour
         return es && es.IsPointerOverGameObject();
     }
 
+    // Quit / play-stop (and standalone exit) fire OnApplicationQuit before teardown; persist
+    // here so the painted maps survive into the next session.
+    private void OnApplicationQuit() => SavePaintOnTeardown();
+
     private void OnDestroy()
     {
+        // Save before tearing the backend down (ReadCanonical needs the live maps). Covers a
+        // scene unload that does not raise OnApplicationQuit; the once-guard prevents a re-save.
+        SavePaintOnTeardown();
+
         if (_bakedMesh) Destroy(_bakedMesh);
         if (_materialInstance) Destroy(_materialInstance);
         if (_proceduralMask) Destroy(_proceduralMask);