ENH: Upstream ConvertAffine interface from fMRIPrep

[oesteban]

Summary

fMRIPrep contains a ConvertAffine interface (fmriprep/interfaces/nitransforms.py, lines 51-101) that converts affine transforms between formats (ITK/FSL/FreeSurfer LTA), with optional inversion. It wraps nitransforms.linear.load() and format-specific I/O.

Transform format conversion is a universal need across all NiPreps tools. This could also live in NiTransforms itself as a Nipype interface wrapper.

Code to upstream

• Interface: ConvertAffine in fmriprep/interfaces/nitransforms.py

Rationale

• Transform format conversion is modality-agnostic
• Currently used in fMRIPrep's init_fsl_bbr_wf
• dMRIPrep, sMRIPrep, and NiBabies can all benefit from the same interface

Priority

HIGH — fundamental transform infrastructure.