Fix paint HUD settings not persisting until a panel is dragged

The debounced auto-save compared painter values against the on-disk
baseline, so after any slider/toggle change the values stayed different
every frame and the 0.5s timer was pushed forward endlessly, never
firing. Saving only happened via the direct drag-end path. Compare
against the previous-frame snapshot instead so the timer fires once the
values settle. Also pin hudSaveId to a stable key so the layout is
shared across scenes instead of split per-scene.

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

Assets/Scenes/Main.unity

@@ -2057,7 +2057,7 @@ MonoBehaviour:
   m_Name: 
   m_EditorClassIdentifier: Assembly-CSharp::PaintHud
   persistHud: 1
-  hudSaveId: 
+  hudSaveId: PaintHUD
   modeManager: {fileID: 0}
   painter: {fileID: 0}
   layoutPrefab: {fileID: 3563111358790320208, guid: fa9dc2ea9e2c9ac4f815f666014804c8, type: 3}