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Add support for NIfTI file format (.nii / .nii.gz)

Add support for NIfTI file format (.nii / .nii.gz): a task in LegoFlow-SWE (Harbor dataset). The method must return a consistent encoded representation (a dictionary with bytes and path keys; at least one of them must be non‑null).

The task

The method must return a consistent encoded representation (a dictionary with `bytes` and `path` keys; at least one of them must be non‑null).

Part of Lego-X/LegoFlow-SWE.