Make the first-grid y padding asymmetric: the bottom (far end of the n_y scan)
can be padded more than the top via daq.auto_raster.grid_padding_fraction_y_
bottom (defaults to grid_padding_fraction_y, so symmetric unless set higher).
The top padding still shifts smargon_top_left; the extra bottom padding only
grows n_y.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>