v9.2.0 — qcow2-bake component + supply-chain/brand housekeeping

New `qcow2-bake` component: converts a promoted bootc image to a qcow2
disk image via bootc-image-builder, in the same standalone-invocation
shape as `installer-anaconda-iso` (separate CI config, web/API/manual
trigger, Package Registry publish). Produces the disk image only;
boot-verifying it needs /dev/kvm and stays a consumer-owned gate on a
self-managed nested-KVM lane. Additive, no consumer behavior change.

Housekeeping in the same release:
- Dropped the dead cosign install (and COSIGN_VERSION/COSIGN_YES) from
  the buildah-fedora builder image; signing left the catalog in v4.3.0
  and cairn's real signing path is the seal component.
- Pinned syft via a syft_version component input instead of fetching
  releases/latest at CI runtime, with a matching Renovate customManager.
- Pinned the remaining floating third-party :latest image refs
  (buildah, skopeo, docker/dind, bootc-image-builder) to version/digest.
- Dropped two unreferenced pre-cairn brand PNGs.