feat(mirax): add support for fluorescence z-stacks

[camlloyd]

Summary

This PR fixes #42 by adding MRXS fluorescence Z-stack support in MiraxReader.

It:

• exposes real Z planes as SizeZ > 1
• excludes ExtFocusLevel from the physical Z axis
• derives PhysicalSizeZ from OFFSET_IN_MICROMETERS
• reads fluorescence focus-page blocks from Index.dat
• preserves the classic path for older/non-stack MRXS files

Validation

The Z-stack behavior was validated with the sample from #42, and also with additional private MRXS Z-stack data.

I also regenerated a full OME-Zarr for the issue sample and checked it visually:

• main Z-stack opened correctly
• label image opened correctly
• fourth channel opened correctly

For regression coverage on non-Z MRXS files, I tested with:

• OpenSlide Mirax test data
• 10.5281/zenodo.10966338 (excluding the BMP24 sample, since BMP24 is not supported on master either)
• 10.5281/zenodo.16962727
• additional private brightfield MRXS data

Focused MiraxReader unit tests cover the new parsing and the classic fallback behavior.

I have not yet validated this branch on JPEG-XR-compressed MRXS data.

Notes

This change was developed with assistance from Codex using GPT-5.4.

One small follow-up left out of this PR to keep the diff focused: indexData in MiraxReader.initFile() still uses explicit close logic rather than try-with-resources.