## Why
After `#13449`, the Linux helper could receive split filesystem and
network policies, but the bubblewrap mount builder still reconstructed
filesystem access from the legacy `SandboxPolicy`.
That loses explicit unreadable carveouts under writable roots, and it
also mishandles `Root` read access paired with explicit deny carveouts.
In those cases bubblewrap could still expose paths that the split
filesystem policy intentionally blocked.
## What changed
- switched bubblewrap mount generation to consume
`FileSystemSandboxPolicy` directly at the implementation boundary;
legacy `SandboxPolicy` configs still flow through the existing
`FileSystemSandboxPolicy::from(&sandbox_policy)` bridge before reaching
bwrap
- kept the Linux helper and preflight path on the split filesystem
policy all the way into bwrap
- re-applied explicit unreadable carveouts after readable and writable
mounts so blocked subpaths still win under bubblewrap
- masked denied directories with `--tmpfs` plus `--remount-ro` and
denied files with `--ro-bind-data`, preserving the backing fd until exec
- added comments in the unreadable-root masking block to explain why the
mount order and directory/file split are intentional
- updated Linux helper call sites and tests for the split-policy bwrap
path
## Verification
- added protocol coverage for root carveouts staying scoped
- added core coverage that root-write plus deny carveouts still requires
a platform sandbox
- added bwrap unit coverage for reapplying blocked carveouts after
writable binds
- added Linux integration coverage for explicit split-policy carveouts
under bubblewrap
- validated the final branch state with `cargo test -p
codex-linux-sandbox`, `cargo clippy -p codex-linux-sandbox --all-targets
-- -D warnings`, and the PR CI reruns
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/13453).
* __->__ #13453
* #13452
* #13451
* #13449
* #13448
* #13445
* #13440
* #13439
---------
Co-authored-by: viyatb-oai <viyatb@openai.com>
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