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/tests/test-specifying-pidns.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+set -xeuo pipefail
+
+srcd=$(cd $(dirname "$0") && pwd)
+. "${srcd}/libtest.sh"
+
+echo "1..1"
+
+# This test needs user namespaces
+if test -n "${bwrap_is_suid:-}"; then
+    echo "ok - # SKIP no setuid support for --unshare-user"
+else
+    mkfifo donepipe
+    $RUN --info-fd 42 --unshare-user --unshare-pid sh -c 'readlink /proc/self/ns/pid > sandbox-pidns; cat < donepipe' >/dev/null 42>info.json &
+    while ! test -f sandbox-pidns; do sleep 1; done
+    SANDBOX1PID=$(extract_child_pid info.json)
+
+    ASAN_OPTIONS=detect_leaks=0 LSAN_OPTIONS=detect_leaks=0 \
+    $RUN --userns 11 --pidns 12 readlink /proc/self/ns/pid > sandbox2-pidns 11< /proc/$SANDBOX1PID/ns/user 12< /proc/$SANDBOX1PID/ns/pid
+    echo foo > donepipe
+
+    assert_files_equal sandbox-pidns sandbox2-pidns
+
+    rm donepipe info.json sandbox-pidns
+
+    echo "ok - Test --pidns"
+fi