Load a portable, tabular X-CNV tree model. The portable artifact is exported
from the pinned model during resource staging and is evaluated without an
XGBoost runtime. Serialized .Rdata, .rds, and native XGBoost files are
deliberately rejected; use tools/stage_xcnv_model.R to convert them in an
environment where XGBoost is installed.
Value
A model object suitable for predict_cnv().