validate: check artifact manifests structurally instead of as images

[RogerHYang]

crane validate fails on any multi-arch image published by BuildKit with provenance/SBOM attestations enabled:

$ crane validate --fast --remote docker.io/arizephoenix/phoenix:version-20.2.0-nonroot
Error: validating children: failed to validate image Manifests[2](sha256:158c42…):
validating config: invalid ConfigFile.RootFS.Type: "" != "layers"

BuildKit attaches attestation manifests to the image index, and recent versions give them the OCI empty descriptor ({}) as their config. validate.Index validates every OCIManifestSchema1/DockerManifestSchema2 child as a container image, so the attestation's config fails the ConfigFile.RootFS.Type == "layers" invariant and validation of an otherwise well-formed image reports failure.

Rather than skipping these children, this change validates them as what they are. A child is treated as an artifact manifest when its index descriptor or manifest declares an artifactType, or its config media type is not an image config (!IsConfig()). Artifact children get structural validation — manifest digest/size/content consistency, config blob existence and digest/size match, layer blob existence (Fast) or digest/size verification (full) — while image-only invariants (rootfs type, diff_ids, platform) are not applied. A registry copy with a missing or corrupt attestation blob still fails validation.

Also fixes two bounds bugs in validateLayers, found while testing with attestation-shaped images:

• the computed-layer slices are appended only for IsLayer() media types but indexed by overall layer position, so an image with a leading non-layer blob panics or compares the wrong layers;
• cf.RootFS.DiffIDs[i] panics with index out of range when the config declares fewer diff_ids than the image has layers (previously reachable via any image whose config is missing rootfs data — now reported as a validation error).

Testing the descriptor-level artifactType path surfaced that mutate.AppendManifests silently dropped IndexAddendum.ArtifactType; it now propagates like the other descriptor overrides.

Tests cover: an index with a BuildKit-style attestation child validates clean (full and Fast); a child detected only via its descriptor artifactType validates clean; a child with a tampered config blob fails on config digest; an image with layers but no diff_ids returns an error instead of panicking.

Verified against a real BuildKit-published image: crane validate --fast --remote docker.io/arizephoenix/phoenix:version-20.2.0-nonroot@sha256:b20fc0… now reports PASS.

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