Second per-scene grid computed with scene_QuadClt=null - the exact camera
model used inside Cuas.transformFromVirtual()'s refinement loop. Its
non-uniformity isolates the refinement residual; its difference from
-NADIR-SCENE-PXPYD isolates the scene-ERS-instance effect. Saved only on
the first (non-undistorted) pass.

Context: 2026-07-10 analysis of the first -NADIR-SCENE-PXPYD dump showed
the sampling grid contains rotation ~0.31x scene roll (R2=0.77) plus
correlated scale (~0.955) and shift_y (to -24 px) concentrated in the
high-roll-rate half of the graveyard segment - explaining the ~0.6x
in-plane content rotation and COLMAP's f=1000/k1=-0.38 self-calibration.
Co-authored-by: Claude <claude@elphel.com>