feat(linux-sandbox): vendor bubblewrap and wire it with FFI (#10413)

## Summary

Vendor Bubblewrap into the repo and add minimal build plumbing in
`codex-linux-sandbox` to compile/link it.

## Why

We want to move Linux sandboxing toward Bubblewrap, but in a safe
two-step rollout:
1) vendoring/build setup (this PR),  
2) runtime integration (follow-up PR).

## Included

- Add `codex-rs/vendor/bubblewrap` sources.
- Add build-time FFI path in `codex-rs/linux-sandbox`.
- Update `build.rs` rerun tracking for vendored files.
- Small vendored compile warning fix (`sockaddr_nl` full init).

follow up in https://github.com/openai/codex/pull/9938

codex-rs/vendor/bubblewrap/SECURITY.md

@@ -0,0 +1,42 @@
+## Security and Disclosure Information Policy for the bubblewrap Project
+
+The bubblewrap Project follows the [Security and Disclosure Information Policy](https://github.com/containers/common/blob/HEAD/SECURITY.md) for the Containers Projects.
+
+### System security
+
+If bubblewrap is setuid root, then the goal is that it does not allow
+a malicious local user to do anything that would not have been possible
+on a kernel that allows unprivileged users to create new user namespaces.
+For example, [CVE-2020-5291](https://github.com/containers/bubblewrap/security/advisories/GHSA-j2qp-rvxj-43vj)
+was treated as a security vulnerability in bubblewrap.
+
+If bubblewrap is not setuid root, then it is not a security boundary
+between the user and the OS, because anything bubblewrap could do, a
+malicious user could equally well do by writing their own tool equivalent
+to bubblewrap.
+
+### Sandbox security
+
+bubblewrap is a toolkit for constructing sandbox environments.
+bubblewrap is not a complete, ready-made sandbox with a specific security
+policy.
+
+Some of bubblewrap's use-cases want a security boundary between the sandbox
+and the real system; other use-cases want the ability to change the layout of
+the filesystem for processes inside the sandbox, but do not aim to be a
+security boundary.
+As a result, the level of protection between the sandboxed processes and
+the host system is entirely determined by the arguments passed to
+bubblewrap.
+
+Whatever program constructs the command-line arguments for bubblewrap
+(often a larger framework like Flatpak, libgnome-desktop, sandwine
+or an ad-hoc script) is responsible for defining its own security model,
+and choosing appropriate bubblewrap command-line arguments to implement
+that security model.
+
+For example,
+[CVE-2017-5226](https://github.com/flatpak/flatpak/security/advisories/GHSA-7gfv-rvfx-h87x)
+(in which a Flatpak app could send input to a parent terminal using the
+`TIOCSTI` ioctl) is considered to be a Flatpak vulnerability, not a
+bubblewrap vulnerability.