fix(deps): bump crossbeam-epoch to 0.9.20 (RUSTSEC-2026-0204) (#311)

Freshly published advisory
[RUSTSEC-2026-0204](https://rustsec.org/advisories/RUSTSEC-2026-0204)
flags `crossbeam-epoch 0.9.18` (invalid pointer dereference in
`fmt::Pointer` for `Atomic`/`Shared`). Transitive dependency;
lockfile-only patch bump to 0.9.20. Currently failing `rust-ci / Cargo
audit` and `Dependency audit` on every PR (first seen on #306).

1174 lib tests green with the bump.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: hyperpolymath