Synthetic mode previously force-overrode the ROI with the full-grid
SYNTH_ROI (320x320). showConvKernel5d expands each ROI pixel by
(vout_dim+1)*sub_dim per axis, so 320x320 blows the heap. Now synthetic
mode uses curt_save_select directly and falls back to SYNTH_ROI only
when no ROI is set, so the user can shrink it to a slice of the grid.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>