feat(registry): materialize verified registry references

[enricopiovesan]

Summary

• persist and validate immutable public contract metadata in synced registry state.
• resolve public registry_ref semver ranges deterministically and cache verified artifacts by digest.

Governing Spec

• 063-registry-contract-materialization

Project Item

• Refs Implement dual-mode capability resolution (registry_ref) + registration-time artifact cache #551

Definition of Done

• Synced public records retain immutable contract URL and digest metadata.
• Range resolution excludes yanked records and reports actionable sync and resolution failures.
• Verified public artifacts use a content-addressed cache.

Validation

cargo fmt --check
cargo test -p traverse-registry public_registry_state