registry: replace go.podman.io/image/v5 with regclient

[crazy-max]

This replaces the remaining go.podman.io/image/v5 usage in the registry stack with github.com/regclient/regclient.

The registry client is now implemented on top of regclient for manifest lookup, digest checks, image config retrieval, and repository tag listing. The old go.podman.io/image/v5 specific transport, logging bridge, and reference handling have been removed in favor of direct regclient integration and smaller package-local helpers where Diun still has behavior that is specific to its model.

This keeps the registry flow daemonless, preserves the existing compare-digest behavior that avoids unnecessary manifest downloads when a matching digest is already stored, and continues to support platform-aware manifest list handling. The migration also keeps Docker config based authentication and explicit registry options working with the new client.

As part of the switch, the registry package was simplified to rely on the parsed Image model directly, duplicate client state was removed, and registry logging now goes through a dedicated regclient logger bridge instead of the old go.podman.io/image/v5 formatter. That restores readable request tracing while matching the transport that is actually in use.