t.to(IoFormat) now relabels any family member as the format instead of refusing (since #315) #343

#315 made family converters rebuild as the class they are handed, including an io subclass (compute/convert.py, module docstring). So converting to a file-format class now succeeds wherever a family converter exists, with no check that the result is valid in that format:

Affine(M).to(NiftyRegAffine)
# before #315 (ec48233): ConversionError "No converter found"
# main (4097ea6):        a NiftyRegAffine

Scaling(..., input=LPSmm(), output=LPSmm()).to(NiftiVoxelToRAS)
# main: a NiftiVoxelToRAS whose endpoints say LPS -> LPS

The format's own conventions (voxel → RAS for NIfTI, NiftyReg's direction and space) are not checked, so the result is a mislabelled format object. io.save would then write it.

Why it matters: #314 (#120) relies on obj.to(Format) either converting exactly or refusing. io.save tries obj.to(fmt) for each candidate format and writes the one that succeeds. With the new behaviour:

• a plain Affine is saved as NiftyReg;
• a Scaling between unknown or LPS systems is saved as NIfTI.

After merging main into #314, two tests fail for this reason: main's own test_io_niftyreg.py::test_plain_affine_is_not_saved_as_niftyreg, and test_io_save.py::test_a_transformation_no_nifti_format_holds_is_refused[scaling].

Options:

1. Keep the generic family converters away from io format classes. Conversion into a format would then need an explicit converter, as Feat(io): convert general transformations exactly into the NIfTI and SPM formats (#120) #314 provides for NIfTI and SPM, and [FEAT] Exact converters into the remaining transformation formats (ITK, NiftyReg, LTA, M3Z, X5, Elastix) #312 for the rest.
2. Keep the relabelling, but limit io.save's conversion pass to formats that have exact converters (this needs a way to tell them apart).
3. Register explicit per-pair converters from every affine-family source into each format. That is many pairs, and it covers only the formats that get them.

This needs a decision before #314 can go green.