chore: append-mode data file; engine -> c6c7ce8 (orig-based stateless/lean)

Runner no longer wipes RESULTS-campaign.md.data on start (incremental reruns
append). Bump benchmark engine submodule to c6c7ce8 so stateless/lean use the
full original prompts. orig + min (r1-4) data preserved; rerunning min once +
stateless/lean ×5 with the new prompts.

run-harness-bench.sh

@@ -174,7 +174,7 @@ run_one() {  # <variant> <rep>
 }
 
 # ---- collect ----
-: > "$DATA"
+# append-mode: data file is NOT wiped (clear manually for a fresh campaign)
 printf 'variant\trep\tsuccess\tbuilder\tadversary\ttotal\tduration_s\tcommits\tloc\n' > "$DATA.hdr"
 for v in "${VARIANTS[@]}"; do
   for rep in $(seq 1 "$REPEATS"); do run_one "$v" "$rep"; done