ci: artifact-based coverage and local Firecracker candidate flow

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

.coveragerc

@@ -1,6 +1,10 @@
 [run]
 branch = True
 source = .
+# Store paths relative to the project root so .coverage.* files produced on
+# different runners (ubuntu-latest vs self-hosted KVM) can be combined by the
+# coverage job without a [paths] remapping section.
+relative_files = True
 
 [report]
 # Coverage policy: see docs/decisions/0004-coverage-policy.md.