Add DICOM loading support

[AssociatedPrimeIdeal]

Goal

Support loading 4D flow MRI data from DICOM inputs and DICOM-converted intermediates.

Tasks

• Add a DICOM loader path, or an initial adapter for the existing DICOM-converted HDF5 format if that is the first integration target.
• Read voxel spacing, orientation, timing, and velocity metadata where available.
• Convert raw DICOM or converted HDF5 data into the normalized internal data model.
• Support converted files that already contain magnitude plus velocity instead of img_complex.
• Detect and handle multi-group converted HDF5 files deterministically.
• Allow loading when segmentation is not embedded.
• Add tests using small anonymized or synthetic DICOM fixtures and converted-HDF5 fixtures.

Acceptance Criteria

• DICOM directories or the agreed DICOM-converted input format can be detected and loaded.
• Loaded data can enter the existing pipeline through the normalized loader model.
• Missing or unsupported metadata produces clear error messages.