bench: add hyperfine wrapper

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

bench/lib/hyperfine.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# hyperfine invocation wrapper.
+
+hyperfine_run() {
+    local out_json="$1" warmup="$2" runs="$3" cmd="$4"
+    hyperfine \
+        --warmup "$warmup" \
+        --runs "$runs" \
+        --export-json "$out_json" \
+        --shell=none \
+        -- "$cmd" >/dev/null
+}
+
+hyperfine_mean_seconds() {
+    jq -r '.results[0].mean' "$1"
+}
+
+hyperfine_stddev_seconds() {
+    jq -r '.results[0].stddev' "$1"
+}