uds: add async Unix socket crate (#18254)

## Summary
- add a codex-uds crate with async UnixListener and UnixStream wrappers
- expose helpers for private socket directory setup and stale socket
path checks
- migrate codex-stdio-to-uds onto codex-uds and Tokio-based stdio/socket
relaying
- update the CLI stdio-to-uds command path for the async runner

## Tests
- cargo test -p codex-uds -p codex-stdio-to-uds
- cargo test -p codex-cli
- just fmt
- just fix -p codex-uds
- just fix -p codex-stdio-to-uds
- just fix -p codex-cli
- just bazel-lock-check
- git diff --check

codex-rs/Cargo.lock

@@ -2891,10 +2891,11 @@ name = "codex-stdio-to-uds"
 version = "0.0.0"
 dependencies = [
  "anyhow",
+ "codex-uds",
  "codex-utils-cargo-bin",
  "pretty_assertions",
  "tempfile",
- "uds_windows",
+ "tokio",
 ]
 
 [[package]]
@@ -3058,6 +3059,18 @@ dependencies = [
  "winsplit",
 ]
 
+[[package]]
+name = "codex-uds"
+version = "0.0.0"
+dependencies = [
+ "async-io",
+ "pretty_assertions",
+ "tempfile",
+ "tokio",
+ "tokio-util",
+ "uds_windows",
+]
+
 [[package]]
 name = "codex-utils-absolute-path"
 version = "0.0.0"
@@ -10970,6 +10983,7 @@ checksum = "9ae9cec805b01e8fc3fd2fe289f89149a9b66dd16786abd8b19cfa7b48cb0098"
 dependencies = [
  "bytes",
  "futures-core",
+ "futures-io",
  "futures-sink",
  "futures-util",
  "pin-project-lite",