Exports *-INTERFRAME.corr-xml scene poses as a COLMAP text sparse model
(PINHOLE camera, empty points3D) so point_triangulator/OpenMVS can use our
IMS/interscene poses with no SfM pose estimation - the FOPEN step-3 path
for vegetation scenes where COLMAP's own mapper fails.

Convention verified against the 2026-07-10 graveyard run (ref
1763232420_274928): R = diag(1,-1,-1) @ (Ry(az)Rx(tilt)Rz(roll))^T,
t = -R C. Fixed-pose triangulation on the corrected NADIR-MERGED-RECTILINEAR
renders gives 0.453 px mean reprojection (COLMAP free mapper: 0.351 px on
the same images).
Co-authored-by: Claude <claude@elphel.com>