docs: complete the per-image-quantity recipe with the HDF5 read-back path

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CLAUDE.md

@@ -203,8 +203,12 @@ A per-image scalar (e.g. `ice_ring_score`, `bkg_estimate`, `mosaicity`) flows an
    in `EndMessage`.
 3. **CBOR**: encode in `frame_serialize/CBORStream2Serializer.cpp` (DataMessage block *and* END block),
    decode in `CBORStream2Deserializer.cpp` (both). Optional fields are back-compatible — no version bump.
-4. **HDF5** (`writer/HDF5DataFilePluginMX.{h,cpp}`): an `AutoIncrVector<float>` with reserve / per-image
-   write / `SaveVector("/entry/MX/<name>")` — per-image arrays go in the data-file plugin, not NXmx master.
+4. **HDF5** write: `writer/HDF5DataFilePluginMX.{h,cpp}` — an `AutoIncrVector<float>` with reserve /
+   per-image write / `SaveVector("/entry/MX/<name>")` (per-image arrays live in the data-file plugin);
+   plus the NXmx master write in `writer/HDF5NXmx.cpp` (`SaveVectorIfMissing(..., end.<name>)`). HDF5
+   **read-back** (so a stored file re-opens, e.g. in the viewer) is in `reader/HDF5MetadataSource.cpp`,
+   NOT `JFJochHDF5Reader.cpp`: mirror the three `bkgEstimate` sites — master `ReadOptVector`, data-file
+   `ReadVector` into the dataset, and the per-image message population.
 5. **Scan result**: `common/ScanResult.h` (`ScanResultElem`) + `common/ScanResultGenerator.cpp`
    (copy in `Add`, resize+fill in `FillEndMessage`).
 6. **Receiver plot**: `common/Plot.h` (`PlotType`) + `common/JFJochReceiverPlots.{h,cpp}` (`StatusVector`