reader: read a third-party NXmx master, and stop a broken one reading as empty

A valid NXmx master written outside the DECTRIS toolchain could not be opened. Measured on a
Diamond-written master of a 360 deg EIGER 16M sweep, where the images and the meta file are pure
DECTRIS and only the master is third-party - which is why the two sides disagree on units at all.
Five independent things, of which two were silent:

* The image size came from detectorSpecific/x_pixels_in_detector, a DECTRIS extension rather than
  NXmx, so a third-party writer has no reason to emit it. It now comes from the image array's own
  shape, as it already did for a VDS master.
* Lengths were assumed to be metres and the units attribute was never read. A pixel size, sensor
  thickness or distance stated in millimetres - correct NXmx - was silently a factor of a thousand
  out. The unit is now read; an undeclared one still means metres, an unknown one is refused.
* The detector distance can sit in NXinstrument rather than in NXdetector; that is now the last
  fallback after the NXmx and the firmware-1.x spellings.
* A pixel mask that is an external link into a file not holding it passed the Exists() check and
  then threw on the open. Whether the array is there is now decided by opening it.
* Each data file was re-opened and searched for /entry/data/data, ignoring the path the master's
  own link names. A master linking to a plain /data therefore found no images at all - and that
  was a warning and exit code 0 over a sweep sitting right there, not an error. The link is now
  taken at its word, and a master that links to data files but yields no images is an error.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3yNBXk4wKdMZy1ak2NY7f

reader/HDF5ImageLocator.cpp

@@ -63,8 +63,8 @@ HDF5ImageLocator::Location HDF5ImageLocator::Resolve(int64_t global_image) const
     if (layout_.format == FileWriterFormat::NXmxLegacy) {
         const uint32_t file_id = global_image / layout_.images_per_file;
         const uint32_t local_index = global_image % layout_.images_per_file;
-        const auto &path = layout_.legacy_files.at(file_id);
-        return {OpenCached(path), local_index, path};
+        const auto &data_file = layout_.legacy_files.at(file_id);
+        return {OpenCached(data_file.path), local_index, data_file.path, data_file.dataset};
     }
 
     if (layout_.format == FileWriterFormat::NXmxVDS
@@ -161,7 +161,8 @@ std::vector<HDF5DataSourceMessage> HDF5ImageLocator::GetSourceMapping(uint64_t f
                 throw JFJochException(JFJochExceptionCategory::HDF5,
                                       "Legacy image source file missing");
 
-            AppendOrExtendSourceMapping(ret, layout_.legacy_files.at(file_id), "/entry/data/data",
+            const auto &data_file = layout_.legacy_files.at(file_id);
+            AppendOrExtendSourceMapping(ret, data_file.path, data_file.dataset,
                                         source_image, local_image, 1);
         }