Parallelize file metadata inspection in 2-stage reader

[Baptiste-Arnould]

Summary

This PR parallelizes file metadata inspection during the initialization of the 2-stage reader.

Previously, metadata for all input files were inspected sequentially by rank 0. For datasets containing a large number of files, this can result in significant initialization time.

The metadata inspection is now distributed across the available reader ranks. The resulting metadata are gathered on rank 0 and reordered according to the original file order before constructing _file_time, _shape, _is_real, and _files_size.

The data-reading algorithm itself is unchanged.

Changes

• Add a helper function to inspect file metadata.
• Distribute metadata inspection across reader ranks.
• Gather metadata on rank 0.
• Restore the original file ordering before constructing reader metadata.

Testing

Tested with MPI on a dataset composed of 11800 files (~250TB in total).

Closes #74