Save the actual scene-frame pXpYD fed to renderGPUFromDSI (after
transformToScenePxPyD and optional undistort) as a pX/pY/D hyperstack at
tile resolution, one frame per scene, suffix -NADIR-SCENE-PXPYD[-RECTILINEAR].
For offline verification of the nadir render geometry: the grid should be
near-uniform (tile centers + ERS displacement); any residual rotation
proportional to scene roll would explain the halved in-plane rotation
observed in the 2026-07-10 graveyard COLMAP test.
Co-authored-by: Claude <claude@elphel.com>