Example/test dataset v1 (IPTS-20267)

IPTS-20267 example/test dataset for NeutronImagingScripts, externalized from
the git tree (it was ~570 MB of plain git objects). Fetched on demand by
neutronimaging.datasets.example_data_dir() via pooch.

• Asset: nis-test-data-v1.tar.gz (295 MB, 936 files: FITS/TIFF frames,
  shutter/spectra sidecars, and the ref_*.npz golden profiles)
• sha256: afb44dbd9875705e271c5199de57f998325220c279b4aa97abdd7392bc7a4773
• Unpacks to the original data/ layout (IPTS-20267/, OB_1_005_*, ...)

Built with COPYFILE_DISABLE=1 tar so no macOS AppleDouble (._*) sidecars
leak into the archive. Bump the tag and the hash in
neutronimaging/datasets.py together if the dataset ever changes (regenerate
goldens via scripts/regenerate_test_refs.py).