Andrey 07/20: A/B must borrow another version dir's merged stack while
STAYING in the current version dir (switching x3d versions rebuilds all
pre-curt files). New saved param curt.det_merged_file (file, directory =
newest *-CUAS-MERGED-CUAS.tiff, bare name = relative to model dir; empty =
the old model-dir scan). Outputs still land in the model dir; fail-loud.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>