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/CBORStream2Serializer.cpp

@@ -620,6 +620,17 @@ inline void CBOR_ENC_START_USER_DATA(CborEncoder& encoder, const char* key,
             break;
     }
 
+    if (message.smargon_position.has_value()) {
+        j["smargon"]["chi_deg"] = message.smargon_position->chi_deg;
+        j["smargon"]["phi_deg"] = message.smargon_position->phi_deg;
+        j["smargon"]["chi_axis"] = {message.smargon_position->chi_axis.x,
+                                    message.smargon_position->chi_axis.y,
+                                    message.smargon_position->chi_axis.z};
+        j["smargon"]["phi_axis"] = {message.smargon_position->phi_axis.x,
+                                    message.smargon_position->phi_axis.y,
+                                    message.smargon_position->phi_axis.z};
+    }
+
     auto str = j.dump();
 
     CBOR_ENC(encoder, key, str);