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

cargo-audit flagged crossbeam-epoch 0.9.18 (RUSTSEC-2026-0204, dated
2026-07-06: invalid pointer dereference in the fmt::Pointer impl for
Atomic/Shared; fixed in >=0.9.20). It is a pre-existing transitive dev
dependency (criterion -> rayon -> crossbeam-deque -> crossbeam-epoch), not
introduced by this branch — advisory-db drift. Root-cause bump to 0.9.20
rather than an ignore, matching the earlier advisory handling.

Verified: `cargo audit --deny warnings` exits 0; `cargo build --workspace`
clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015qwVESTcbfanY2iJPQNoSz

Author: claude