XGB gamma/hadron C++ interface hard-wired to `Is_Gamma_70`

[GernotMaier]

Location: EventDisplay_v4/src/CData.cpp:1118-1127; EventDisplay_v4/src/VGammaHadronCuts.cpp:727-731,1118-1128
The C++ consumer binds only Gamma_Prediction and Is_Gamma_70; the classification operating
point is baked into the upstream friend-tree producer, not enforced by VGammaHadronCuts.
Impact: The 70% efficiency threshold cannot be adjusted from cut files or runtime parameters,
discarding all the optimisation flexibility of the XGB classifier.