reader: open a master whose beamline writes standards-correct NXmx differently

Four defects, hit in sequence, that between them stopped eight masters from one beamline before any
geometry question was reached. The files are correct NeXus; the reader was assuming one writer's
conventions.

ReadScalar demanded rank 0. NXmx puts no rank on distance, saturation_value, two_theta or det_z, and
these files write them as shape (1,). Any dataspace holding exactly one element is now accepted; a
genuine vector is still refused.

frame_time was read unconditionally and NXmx does not require it.

A virtual-dataset source filename of "." was resolved as a relative path, giving <dir>/. - but "."
is HDF5's spelling for THIS file, and these masters compose /entry/data/data as a virtual dataset
over datasets in themselves that are external links to the data files.

The virtual source's DATASET PATH was parsed and then ignored in favour of a hardcoded
/entry/data/data, while these data files keep their images at the root. Fixing that exposed a fifth:
the positional-read fast path used the master's own path, but a dataset reached through an external
link lives in another file and a chunk address is an offset into THAT file.

An audit of all 38 masters in the non-SLS corpus finds exactly these eight need the change and the
other 30 need nothing. Corpus A/B over 73 dataset pairs, base and patched back to back: 64
byte-identical results, 9 identical failures, none differing. The blast radius is bounded by
construction - JFJochReader is linked only by rugnux and jfjoch_viewer, and the one shared header
this touches only ever accepts more, so nothing that opened before can read differently.

With it the eight files read completely: 12850 images, no decode errors, seven of them spanning two
source datasets through the master's own external links.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Lc5JG6kJqZoCWaoZ43JGTW

reader/HDF5ImageLocator.cpp

@@ -74,7 +74,7 @@ HDF5ImageLocator::Location HDF5ImageLocator::Resolve(int64_t global_image) const
             if (!mapping.ContainsVirtualImage(image))
                 continue;
             return {OpenCached(mapping.filename), static_cast<uint32_t>(mapping.SourceImage(image)),
-                    mapping.filename};
+                    mapping.filename, mapping.dataset};
         }
         throw JFJochException(JFJochExceptionCategory::HDF5,
                               "Image not covered by /entry/data/data VDS mappings");