remote: verify schema1 digest pulls by body hash

[1seal]

summary

• validate schema1 digest pulls against the returned manifest body hash instead of trusting Docker-Content-Digest
• preserve the historical schema1 header-based digest behavior for tag pulls to avoid breaking older registries
• add regression coverage for both digest-pull validation and schema1 tag compatibility

root cause

for schema1 signed manifests, fetchManifest computed the body digest and then replaced it with Docker-Content-Digest before checking digest references. that meant digest pulls could succeed based on the header value instead of the manifest bytes that were actually returned.

testing

• go test ./pkg/v1/remote