ENH: Upstream Clip and Label2Mask math interfaces from fMRIPrep

[oesteban]

Summary

fMRIPrep contains two general-purpose image math interfaces in `fmriprep/interfaces/maths.py`:

`Clip` (lines 21-52)

Clips NIfTI image values to a specified min/max range. If no values are outside bounds, passes through without copying.

`Label2Mask` (lines 64-84)

Extracts a binary mask from a multi-label segmentation for a specific label value.

Both are completely general image operations with no fMRI specificity. They could live in `niworkflows.interfaces.nibabel` or a new `niworkflows.interfaces.maths` module.

Code to upstream

• `Clip` in `fmriprep/interfaces/maths.py`
• `Label2Mask` in `fmriprep/interfaces/maths.py`

Rationale

• General-purpose NIfTI manipulation needed by any tool working with parametric maps or segmentation labels
• Used in fMRIPrep's T2* reporting but applicable across modalities

Priority

MEDIUM.