oven: add QEMU smoke test between build and upload
Runs `ac-os test` after kernel build, before CDN upload.
Boots kernel in QEMU headless, checks serial for boot success
or kernel panic. Non-fatal for now (logs warning, continues upload)
until virtio-gpu is validated. Adds smoke-test stage to progress.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>