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

frame_serialize/CBORStream2Serializer.cpp

@@ -238,6 +238,8 @@ inline void CBOR_ENC(CborEncoder &encoder, const SpotToSave& spot) {
     CBOR_ENC(mapEncoder, "indexed", spot.indexed);
     CBOR_ENC(mapEncoder, "latt", spot.lattice);
     CBOR_ENC(mapEncoder, "image", spot.image);
+    if (spot.panel >= 0)
+        CBOR_ENC(mapEncoder, "panel", static_cast<int64_t>(spot.panel));
     if (spot.indexed) {
         CBOR_ENC(mapEncoder, "h", spot.h);
         CBOR_ENC(mapEncoder, "k", spot.k);
@@ -484,6 +486,33 @@ inline void CBOR_ENC_AZINT_MAP(CborEncoder &encoder, const StartMessage &msg) {
     CBOR_ENC_2D_TYPED_ARRAY(encoder, image);
 }
 
+// Not a DECTRIS field. Absent for a flat detector (no panel table), which is what every stream written
+// before the key existed means.
+inline void CBOR_ENC_DETECTOR_MODULES(CborEncoder &encoder, const std::vector<DetectorModule> &modules) {
+    if (modules.empty())
+        return;
+    CborEncoder arrayEncoder;
+    cborErr(cbor_encode_text_stringz(&encoder, "detector_modules"));
+    cborErr(cbor_encoder_create_array(&encoder, &arrayEncoder, modules.size()));
+    for (const auto &m: modules) {
+        CborEncoder mapEncoder;
+        cborErr(cbor_encoder_create_map(&arrayEncoder, &mapEncoder, 10));
+        CBOR_ENC(mapEncoder, "x0", m.x0);
+        CBOR_ENC(mapEncoder, "y0", m.y0);
+        CBOR_ENC(mapEncoder, "width", m.width);
+        CBOR_ENC(mapEncoder, "height", m.height);
+        const std::pair<const char *, double> placement[] = {
+            {"ox", m.ox}, {"oy", m.oy}, {"fx", m.fx}, {"fy", m.fy}, {"sx", m.sx}, {"sy", m.sy}
+        };
+        for (const auto &[key, val]: placement) {
+            cborErr(cbor_encode_text_stringz(&mapEncoder, key));
+            cborErr(cbor_encode_double(&mapEncoder, val));
+        }
+        cborErr(cbor_encoder_close_container(&arrayEncoder, &mapEncoder));
+    }
+    cborErr(cbor_encoder_close_container(&encoder, &arrayEncoder));
+}
+
 inline void CBOR_ENC_ROI_MAP(CborEncoder &encoder, const StartMessage &msg) {
     if (msg.roi_map.empty())
         return;
@@ -835,6 +864,7 @@ void CBORStream2Serializer::SerializeSequenceStart(const StartMessage& message)
     CBOR_ENC_PIXEL_MASK(mapEncoder, message);
     CBOR_ENC_AZINT_MAP(mapEncoder, message);
     CBOR_ENC_ROI_MAP(mapEncoder, message);
+    CBOR_ENC_DETECTOR_MODULES(mapEncoder, message.detector_modules);
 
     CBOR_ENC(mapEncoder, "channels", message.channels);
     CBOR_ENC(mapEncoder, "max_spot_count", message.max_spot_count);