fix(deps): bump crossbeam-epoch to 0.9.20 (RUSTSEC-2026-0204) (#359)

RustSec published RUSTSEC-2026-0204: an invalid pointer dereference in the
fmt::Pointer impl for crossbeam-epoch's Atomic and Shared when the underlying
pointer is invalid. It reaches us transitively via crossbeam-deque. The fix is
a lock-only patch bump to the already-compatible 0.9.20 (no Cargo.toml or code
change). cargo-deny's advisories check now passes.

Cargo.lock

@@ -277,9 +277,9 @@ dependencies = [
 
 [[package]]
 name = "crossbeam-epoch"
-version = "0.9.18"
+version = "0.9.20"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5b82ac4a3c2ca9c3460964f020e1402edd5753411d7737aa39c3714ad1b5420e"
+checksum = "2d6914041f254d6e9176c01941b21115dcfb7089e55135a35411081bd106ef3f"
 dependencies = [
  "crossbeam-utils",
 ]