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/network.h

@@ -0,0 +1,22 @@
+/* bubblewrap
+ * Copyright (C) 2016 Alexander Larsson
+ * SPDX-License-Identifier: LGPL-2.0-or-later
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#pragma once
+
+void loopback_setup (void);