chore(engine): bump crossbeam-epoch 0.9.18 -> 0.9.20 (RUSTSEC-2026-0204)

Invalid pointer dereference in fmt::Pointer for Atomic/Shared null
pointers; flagged by cargo deny in the pre-push gate. Transitive via
rayon; lockfile-only change.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/Cargo.lock

@@ -534,9 +534,9 @@ 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",
 ]