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/Dockerfile

@@ -0,0 +1,15 @@
+# perfbench container — #709. Same toolset as the fleet LXC/guest installer.
+# Build: docker build -t git.knownelement.com/reachableceo/perfbench:1.0 .
+FROM debian:12-slim
+
+RUN apt-get update -qq && apt-get install -y -qq --no-install-recommends \
+        fio iperf3 jq ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY bench-run.sh /usr/local/bin/bench-run.sh
+RUN chmod +x /usr/local/bin/bench-run.sh
+
+# storage bench: mount the path to test at /mnt/bench
+# network bench: run as server: docker run ... bench-run.sh --server
+ENV BENCH_DIR=/mnt/bench
+ENTRYPOINT ["/usr/local/bin/bench-run.sh"]