conformance: add vcluster leg to the CI matrix

[jleni]

Split from #10 (deliberately deferred — hardening the working k3s path first).

The vcluster backend has no e2e harness pool yet, so it is a documented exemption in backend_conformance_coverage_is_classified (crd/profile.rs). Before vcluster carries production load it must join the conformance matrix:

• add a vcluster pool to hack/e2e.ts
• add a just test-smoke-vcluster recipe
• add the matrix leg in .github/workflows/ci.yml and flip its classification in the enforcement test

Same story applies to k0s and capi (capi additionally needs a CI infrastructure provider).