feat(perf): standardized fleet benchmark artifact (fio/iperf3) [#709]

bench-run.sh: identical test shape everywhere (4k rand rw, 1M seq rw,
direct IO, grep-able BENCH| lines) - runs in guests, LXCs, containers.
install-bench.sh for Debian guests/LXCs; Dockerfile for registry image;
lxc-bench-setup.sh stands up the host-side bench LXC (proxmox = no docker).

Ticket: https://projects.knownelement.com/issues/709

proxmox/perf/bench/install-bench.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+# install-bench.sh — install the #709 benchmark toolset (fio + iperf3 + jq)
+# on a Debian-based guest or LXC. Idempotent. Not for Proxmox hosts directly:
+# on hosts, create the bench LXC (see lxc-bench-setup.sh) and run this inside.
+set -eu
+export DEBIAN_FRONTEND=noninteractive
+apt-get update -qq
+apt-get install -y -qq fio iperf3 jq >/dev/null
+echo "bench toolset installed: fio=$(fio --version 2>/dev/null | awk '{print $3}') iperf3=$(iperf3 --version 2>/dev/null | awk 'NR==1{print $2}')"