fix(deps): bump crossbeam-epoch to 0.9.20 for RUSTSEC-2026-0204 (#162) (#163)

RUSTSEC-2026-0204 (invalid pointer dereference in crossbeam-epoch's
fmt::Pointer impl for Atomic/Shared) hard-fails cargo audit on every branch,
since the advisory's addition to the RustSec db. crossbeam-epoch is a fully
transitive dependency (moka -> hickory-resolver -> libp2p-dns), and the
advisory's prescribed fix is >= 0.9.20; moka 0.12.15 accepts >=0.9.18,<0.10.0
so the bump resolves with no other graph changes.

Lockfile-only, single crate. Verified: cargo audit exits 0 and the workspace
builds clean with 0.9.20.

Closes #162.

Co-authored-by: t <t@t>

Author: beardthelion