smargon: write chi/phi goniometer position into NXmx transformations

Add an optional Smargon static positioner (chi/phi angles + rotation axes)
that is reconstructed into the NXmx sample transformation chain. Chi/phi are
appended at the innermost end of the chain (closest to the sample) for both
the goniometer and grid-scan branches, with axes defaulting to chi {0,0,1}
and phi = omega default {1,0,0}.

- SmargonPosition gains chi_axis/phi_axis (common/JFJochMessages.h)
- OpenAPI: optional phi_axis/chi_axis arrays; clients regenerated
- OpenAPIConvert wires Dataset_settings.smargon -> DatasetSettings
- CBOR serializer/deserializer round-trip the axes
- tests: CBORSerialize_Start_Smargon

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frame_serialize/CBORStream2Deserializer.cpp

@@ -1161,7 +1161,28 @@ namespace {
                 else
                     message.geom_refinement_algorithm = GeomRefinementAlgorithmEnum::None;
             }
+            if (j.contains("smargon")
+                && j["smargon"].is_object()
+                && j["smargon"].contains("chi_deg")
+                && j["smargon"].contains("phi_deg")
+                && j["smargon"]["chi_deg"].is_number()
+                && j["smargon"]["phi_deg"].is_number()) {
 
+                SmargonPosition smargon;
+                smargon.phi_deg = j["smargon"]["phi_deg"].get<float>();
+                smargon.chi_deg = j["smargon"]["chi_deg"].get<float>();
+                if (j["smargon"].contains("chi_axis") && j["smargon"]["chi_axis"].is_array()
+                    && j["smargon"]["chi_axis"].size() == 3)
+                    smargon.chi_axis = {j["smargon"]["chi_axis"][0].get<float>(),
+                                        j["smargon"]["chi_axis"][1].get<float>(),
+                                        j["smargon"]["chi_axis"][2].get<float>()};
+                if (j["smargon"].contains("phi_axis") && j["smargon"]["phi_axis"].is_array()
+                    && j["smargon"]["phi_axis"].size() == 3)
+                    smargon.phi_axis = {j["smargon"]["phi_axis"][0].get<float>(),
+                                        j["smargon"]["phi_axis"][1].get<float>(),
+                                        j["smargon"]["phi_axis"][2].get<float>()};
+                message.smargon_position = smargon;
+            }
             if (j.contains("ring_current_mA"))
                 message.ring_current_mA = j["ring_current_mA"];
             if (j.contains("sample_temperature_K"))