oci 1.1 referrers selection is order-dependent (last element)

[1seal]

hi, i noticed the oci 1.1 referrers discovery paths select the last referrer returned when multiple results exist, which makes behavior dependent on registry response order.

examples at commit 03be625:

• pkg/cosign/verify.go (oci 1.1 signature discovery): warns on multiple results, then selects results[numResults-1]
• pkg/oci/remote/remote.go (attachmentExperimentalOCI): warns on multiple results, then selects results[numResults-1]

since referrers ordering is not guaranteed, this can lead to non-deterministic behavior across registries and over time when multiple referrers exist for the same artifactType.

question:
what behavior should cosign prefer when len(index.Manifests) > 1 for a requested artifactType?

• fail closed as ambiguous?
• fetch/verify all matching referrers?
• deterministic selection using a specific key (e.g. validated annotations), and if so which key?

i can send a small PR that at least makes selection deterministic (by sorting descriptors before selecting), but wanted to confirm the intended semantics first.