Default USE_FLAGTUNE to 0 and add fold_scale A/B script with trace shapes

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/.

.gitignore

@@ -1,2 +1,3 @@
-runs/
+runs/*
+!runs/shape/
 __pycache__/