bench: wasm-opt benchmark harness — Hetzner config sweeps + local QEMU VM

Tooling that produced the perf findings (committed for rerunnability):
- scripts/bench/wasm-opt-bench.sh + o2-config-sweep.sh + sweep.conf:
  replay asyncify/-O2 over a cached fixture under allocator/THP/core
  matrices on the Hetzner runner.
- scripts/bench/setup-vm.sh + cloud-init/ + vm-build.sh: local QEMU
  (HVF) Ubuntu guest with Docker CE to verify Linux builds without
  burning paid runners.
- .gitignore: bench fixtures/results and the VM image stay local.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -67,3 +67,9 @@ output/
 # Curated design docs live under docs/features/ and ARE committed.
 /features/
 
+# wasm-opt benchmark local artifacts. The harness scripts under scripts/bench/
+# ARE committed; the fixtures/results (/bench) and the QEMU VM image
+# (scripts/bench/vm) are local-only.
+/bench/
+/scripts/bench/vm/
+