Generates a multi-slice float32 TIFF (NaN-padded, ImageJ-compatible):
  Slice 1: tile-processor disparity D scattered to nadir image pixels
           (from *-NADIR-DISPARITY-MAPS.tiff, averaged across all scenes)
  Slice 2: COLMAP sparse Z_cam projected to the same nadir pixel grid
           (optional --dense PLY for OpenMVS dense cloud)
  Slice 3: spatial superposition (tile-proc priority; COLMAP where missing;
           average at overlapping pixels)
  Slice 4: tile-processor scene-count per pixel (--counts flag)

Uses the COLMAP reference-image pose (auto-detected as centroid of camera
centres, or specified via --ref-image) to project 3D points. Handles
OPENCV lens distortion model (--no-distortion to skip).

Intended for evaluating depth-fusion approaches (tile processor + SIFT):
  python3 scripts/fopen_nadir_compare.py \
      --disp   <ts>-NADIR-DISPARITY-MAPS.tiff \
      --colmap sparse/0_txt/ [--dense scene_dense.ply] \
      --out    nadir_compare.tif --counts
Co-authored-by: Claude <claude@elphel.com>