Add image re-encoding benchmarks.

This is one of the more compute-heavy workflows that the
app server performs, we should be able measure how fast it is
against past revisions.

Add a step to the general CI job which will run each benchmark
as a test without trying to actually measure many iterations.

scripts/smoke-test-rust-benches.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+cd "$(dirname "${BASH_SOURCE[0]}")/../codex-rs"
+
+bench_targets="$(
+  cargo metadata --no-deps --format-version 1 \
+    | jq -r '.packages[] as $package | $package.targets[] | select(any(.kind[]; . == "bench")) | [$package.name, .name] | @tsv'
+)"
+
+if [[ -n "$bench_targets" ]]; then
+  while IFS=$'\t' read -r package bench_target; do
+    cargo bench -p "$package" --bench "$bench_target" -- --test
+  done <<< "$bench_targets"
+fi