Add API to resolve manifest for current platform

Add open_image_this_platform() which resolves a manifest from an OCI
directory for the native platform (OS and architecture). This handles
manifest lists by peeling them one level to find platform-specific
manifests.

Returns ResolvedManifest containing:
- The resolved manifest and its digest
- The source image index and digest if resolution went through a
  manifest list

Assisted-by: OpenCode (claude-opus-4-5)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters