Each test job now runs once under coverage and uploads a small .coverage.*
artifact. The coverage job combines them on ubuntu-latest — no test reruns,
no KVM dependency. The infra candidate is built directly on the KVM runner,
eliminating the build-infra job and the ~70 s upload + ~83 s combined
download. For PRs, no rootfs artifact is transferred at all. Main-branch
pushes upload the tested rootfs and matching dropbear so publish-infra
publishes the byte-identical artifact. relative_files = True in .coveragerc
lets coverage files from different runners combine without path remapping.
Closes#446
