Plain autotune is the common case (fast, same-config A/B); FlagTune's
exhaustive first-run search is now opt-in via USE_FLAGTUNE=1.
ab_fold_test.sh automates the FLAGGEMS_MXFP4_FOLDSCALE=0/1 record-replay
comparison over the four DeepSeek-V4-Flash trace shape sets, which are
now tracked under runs/shape/.