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.
