e2e tier-vm-qemu savevm boot-cache is dead: actions/cache stores only vm-overlay.qcow2, never the cache-key sidecar the CLI requires for a warm resume (#49 follow-up)

[Xiddoc]

Found by an automated codebase audit (follow-up on #49, shipped in #248).

The "Restore/persist the booted savevm overlay" step caches path: vmphone/vm-overlay.qcow2 only (.github/workflows/e2e.yml:439). But the warm-resume path (src/beetroot/backends/vm.py:545 → vm/boot_cache.py:overlay_is_stale) treats a missing/unreadable vm-overlay.cache-key sidecar as stale. The sidecar lives next to the overlay but is never cached, so every run re-cold-boots and the savevm cache delivers no speedup — the #49 mechanism is effectively dead in CI.

Failure scenario

Run tier-vm-qemu twice with unchanged kernel+rootfs. Run 1: cache miss → cold boot → post-job saves vm-overlay.qcow2 (not the sidecar). Run 2: cache hit restores vm-overlay.qcow2 with no vm-overlay.cache-key alongside → overlay_is_stale() returns True (missing sidecar) → cold boot again.

Suggested fix

Add the sidecar to the cached path:

path: |
  vmphone/vm-overlay.qcow2
  vmphone/vm-overlay.cache-key

Also note scripts/vm_cache_key.py (kernel+rootfs only) omits the resolved -smp/-m geometry that boot_cache.base_identity folds in (#161), so even with the sidecar cached, the CI cache key and the CLI staleness fingerprint can disagree — worth reconciling.

Location: .github/workflows/e2e.yml:439

Severity: med · category: bug (ci)