feat(bench): PLAIN_ONLY knob — skip causal cells for concurrency sweeps

The c=64 matrix run (job 7dbc1ede) showed plain-mode parity but a 2.2x ka
latency penalty; the decisive test is a CONNS sweep, which only needs the
plain cells. PLAIN_ONLY=1 skips the two causal cells; the summary already
tolerates their absence.

scripts/ka-close-matrix.sh

@@ -144,8 +144,10 @@ run_causal() { # $1=mode $2=port
 # ---- matrix --------------------------------------------------------------
 run_plain  ka    "$BASE_PORT"
 run_plain  close "$((BASE_PORT + 1))"
+if [ "${PLAIN_ONLY:-0}" != 1 ]; then
   run_causal ka    "$((BASE_PORT + 2))"
   run_causal close "$((BASE_PORT + 3))"
+fi
 
 # ---- summary -------------------------------------------------------------
 say "=== SUMMARY ==="