bench: fix perf_stat_metric to handle :u event-name suffixes

perf stat appends user/kernel/hypervisor modifiers (:u/:k/:h) to event
names when paranoid >= 2, which is the documented configuration. Strip
that suffix before comparison so 'instructions' matches 'instructions:u'.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

bench/lib/perf.sh

@@ -26,7 +26,11 @@ perf_stat_metric() {
     awk -F',' -v ev="$event" '
         # skip comment/header lines starting with #
         /^#/ { next }
-        $3 == ev { print $1; found=1; exit }
+        {
+            e = $3
+            sub(/:[ukh]+$/, "", e)
+            if (e == ev) { print $1; found=1; exit }
+        }
         END { if (!found) print "null" }
     ' "$csv"
 }