Update M6 manifest building to register T1c to CT with rigid
Mattes MI on a CT-space head box, including multi-start initial
tilts for large inter-session head motion. The inverse transform
warps CT and GTV into T1c native space, with acceptance based on
concentricity of the warped CT skull ring and MR head silhouette.
Rerun preprocessing when a row has a label but the warped label file
is missing. Normalize volume orientation in read_nii_img.
Add registration benchmark documentation, benchmark script, and
shared registration pipeline helpers.
