fix: bump crossbeam-epoch in fuzz lockfile for RUSTSEC-2026-0204

The fuzz/ directory is a separate crate with its own Cargo.lock, and the
Dependency Audit job audits it separately (cd fuzz && cargo audit). The
previous commit only updated the root lockfile, so that second audit
step still failed on RUSTSEC-2026-0204.

These are the only two lockfiles in the repo, and both are now at 0.9.20.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: berisher