feat(build): cosign-verify + digest-pin pre-bundled control-plane images (ADR-16 P5)

[wrkode]

What

Harden the air-gap image bundle (ADR-16 P5, the deferred security-gated slice of
kairos-io/kairos#4205). The pre-bundled control-plane images land in the immutable OS
and are ctr imported at boot, bypassing any runtime CRI image policy — so provenance
is established at build time via cosign signature verification + digest pinning.

How

The image-bundler stage, for each image kubeadm requires:

1. resolves the floating tag to an immutable digest once (crane digest),
2. cosign-verifies that digest against the Kubernetes krel keyless identity
   (krel-trust@k8s-releng-prod.iam.gserviceaccount.com / https://accounts.google.com),
3. pulls the same digest (crane pull ref@digest) — verify and pull are bound to
   one digest (TOCTOU-safe).

cosign and crane are version-pinned and checksum-verified at install.

Best-effort verification with a hard floor

registry.k8s.io signature coverage is empirically incomplete: kube-apiserver,
kube-controller-manager, and kube-proxy carry no krel signature (confirmed by
identical digest + absent .sig/referrers at the canonical us-central1-docker.pkg.dev
backend — a known upstream condition, k8s#129199), while kube-scheduler, coredns,
pause, etcd do verify. So:

• signature verifies → recorded verified: true.
• cosign reports exactly no signatures found → recorded verified: false / no-upstream-signature (digest-pinned only; still content-addressed).
• any other cosign failure (present-but-invalid signature, network/Fulcio/Rekor/TUF
  error) → hard fail.
• floor (build-blocking): pause + etcd + coredns AND ≥1 kube-* component MUST
  verify — each a positive pass (a timeout can't satisfy the floor).

Auditability

A baked /opt/provider-kubernetes/images/images.lock records ref → digest plus the
per-image verified flag/reason. The release pipeline converts it to a CycloneDX SBOM
(build/gen-bundled-sbom.py) and attests it, so each released image attests exactly
which control-plane images it baked and at what trust level (ADR-15).

Validation

• Full make image green; the bundler stage exercised cosign against registry.k8s.io
  for real → 7 images, 4 signature-verified, 3 recorded no-upstream-signature.
• images.lock + 7 tarballs confirmed in the final image; sh -n + shellcheck clean;
  release YAML valid; Go gates green.

Security review

security-architect gate: PASS — B1a/B1b/B1c (with R1: precise no-signature match,
R2: positive-pass floor), B2–B7 all verified in code. Residual risk recorded
(R-resid-1: the kube-* trio is digest-pinned-only and upstream-unsigned; B1a
auto-tightens to require their signatures if/when upstream starts signing them.
R-resid-2: backlog CI canary to detect that change). Two should-fix nits actioned
inline (anchored floor matches; documented closed-enum verifyReason).

[wrkode]

LGTM