feat(linux-sandbox): implement proxy-only egress via TCP-UDS-TCP bridge (#11293)

## Summary
- Implement Linux proxy-only routing in `codex-rs/linux-sandbox` with a
two-stage bridge: host namespace `loopback TCP proxy endpoint -> UDS`,
then bwrap netns `loopback TCP listener -> host UDS`.
- Add hidden `--proxy-route-spec` plumbing for outer-to-inner stage
handoff.
- Fail closed in proxy mode when no valid loopback proxy endpoints can
be routed.
- Introduce explicit network seccomp modes: `Restricted` (legacy
restricted networking) and `ProxyRouted` (allow INET/INET6 for routed
proxy access, deny `AF_UNIX` and `socketpair`).
- Enforce that proxy bridge/routing is bwrap-only by validating
`--apply-seccomp-then-exec` requires `--use-bwrap-sandbox`.
- Keep landlock-only flows unchanged (no proxy bridge behavior outside
bwrap).

---------

Co-authored-by: Codex <199175422+chatgpt-codex-connector[bot]@users.noreply.github.com>

codex-rs/linux-sandbox/src/lib.rs

@@ -10,6 +10,8 @@ mod landlock;
 #[cfg(target_os = "linux")]
 mod linux_run_main;
 #[cfg(target_os = "linux")]
+mod proxy_routing;
+#[cfg(target_os = "linux")]
 mod vendored_bwrap;
 
 #[cfg(target_os = "linux")]