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

[beardthelion]

Bumps crossbeam-epoch from 0.9.18 to 0.9.20 to clear RUSTSEC-2026-0204 (invalid pointer dereference in its fmt::Pointer impl for Atomic/Shared), which is hard-failing cargo audit on every branch since the advisory landed in the RustSec db.

crossbeam-epoch is a fully transitive dependency (moka -> hickory-resolver -> libp2p-dns); the advisory's prescribed fix is >= 0.9.20, and moka 0.12.15 accepts >=0.9.18, <0.10.0, so the bump resolves with no other graph changes. Lockfile-only, single crate.

This lands on main, so it clears the red cargo audit for every open PR at once.

Verified by execution:

• cargo audit: exit 1 (RUSTSEC-2026-0204) before, exit 0 after.
• cargo build --workspace: compiles clean with 0.9.20.
• cargo test --workspace: 829 pass, 0 fail.

Closes #162.

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[jatmn]

Thanks for the contribution. I do not see any actionable issues from my review.

@kevincodex1 LGTM