Browse… writes the chosen picture into BOTH CustomBackgroundPath and AssetId, and the default
stamp bails when AssetId is already set. So flipping "Use default background" back on changed a
bool while the preview kept showing the custom image — it looked like the switch did nothing.
Turning the default on now clears the custom pointer and the asset, re-runs
StampDefaultBackgroundAsset for the scene's shipped art, and only then invalidates the bake,
updates the preview and refreshes thumbnails.
BackgroundTests pass (17); Release 0 warnings.