Panel id on spots and the panel table in the start message

SpotToSave gains int16_t panel (-1 = no panel table). It sits in the padding
after the two bools, so sizeof(SpotToSave) stays 80. It is serialised only
where it is set: CBOR spot key "panel", HDF5 /entry/MX/peakPanel (data file,
plus a virtual dataset in a VDS master when the start message has panels),
read back by HDF5MetadataSource. For a flat detector none of these is written,
so files and streams for existing data are byte-for-byte unchanged.

The start message carries the panel table (detector_modules, CBOR key absent
when empty), filled from the experiment; JFJochHttpReader takes it, so a
stream viewer knows the panels too.

AssignModules takes the spot's panel id where it has one - the stored pixel box
it was found in - and falls back to the plane position otherwise (every spot
until per-panel spot finding sets the id).

Reflection deliberately gets no panel id (owner decision): it is needed only
between prediction and integration, and the struct feeds GPU layout and
memory budgets.

Tests: CBORSerialize_SpotPanelId, CBORSerialize_StartDetectorModules,
JFJochReader_PanelId (VDS/legacy/integrated, with and without panels).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015CeEGgpKdfkCTJb3YNMJcu

reader/HDF5MetadataSource.cpp

@@ -1291,6 +1291,13 @@ static void ReadSpotsFromFiles(HDF5Object &master_file,
         {master_image, 0}, {source_image, 0}, {1, spot_count}
     );
 
+    // Written only for a detector with a panel table.
+    auto spot_panel = ReadVectorMasterFirst<int16_t>(
+        master_file, source_file,
+        "/entry/MX/peakPanel",
+        {master_image, 0}, {source_image, 0}, {1, spot_count}
+    );
+
     message.spots.reserve(message.spots.size() + spot_count);
     for (size_t i = 0; i < spot_count; i++) {
         const auto x = spot_x.at(i);
@@ -1317,6 +1324,8 @@ static void ReadSpotsFromFiles(HDF5Object &master_file,
             s.ice_ring = (spot_ice.at(i) != 0);
         if (spot_lattice.size() > i)
             s.lattice = spot_lattice.at(i);
+        if (spot_panel.size() > i)
+            s.panel = spot_panel.at(i);
         message.spots.emplace_back(s);
     }