Function verify_release 

pub fn verify_release(
    client: &GitHubClient,
    owner: &str,
    repo: &str,
    base_tag: &str,
    head_tag: &str,
    policy: Option<&str>,
    with_evidence: bool,
) -> Result<VerificationResult>

Verify a release (tag range) and return a verification result.

This encapsulates the full release verification flow: compare refs, resolve commit PRs, collect attestations, build bundle, assess.