[nvaytet]

The order in which masks are saved in the nexus files is backwards: https://github.com/scipp/ess/blob/main/packages/essreduce/src/ess/reduce/masking.py#L172

It’s getting name from the top level, which should be 'loki_detector_0', but then creates the NXdetector using bank which is actually 'bad_tubes' in my case.

Masks should have this structure to start with:
masks = {'loki_detector_0': {'bad_tubes': sc.array()}}

This is how the ADR reads:

/                                  attributes file_time, creator and creator_version
/entry                             NXentry
    valid_from                     "2026-11-01T00:00:00Z", not read by workflows
    instrument                     NXinstrument
        name                       "LOKI"
        loki_detector_0            NXdetector
            masks                  NXcollection
                broken_tubes       uint8, HDF5 dimension labels ("tube",)
                beamstop           uint8, HDF5 dimension labels ("layer", "tube", "straw")
        loki_detector_1            NXdetector
            masks                  NXcollection
                broken_tubes       ...

The result in the nexus file is that the NXdetector is called bad_tubes.

In this PR we invert the logic and save the masks as intended.

[jl-wynen]

Seems like we need to not approve code that we didn't read thoroughly