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

common/DiffractionExperiment.cpp

@@ -652,6 +652,7 @@ SpotFindingSettings DiffractionExperiment::DefaultDataProcessingSettings() {
 
 void DiffractionExperiment::FillMessage(StartMessage &message) const {
     message.images_per_file = GetImagesPerFile();
+    message.detector_modules = GetModules().Get();
     message.beam_center_x = GetBeamX_pxl();
     message.beam_center_y = GetBeamY_pxl();
     const auto direct_beam = GetDiffractionGeometry().GetDirectBeam_pxl();