## Why
Preparing a filesystem sandbox could synchronously probe unrelated permission
roots on the executor runtime thread and repeatedly resolve the same filesystem
aliases while deriving writable roots.
## What changed
- On Linux, leave permission-root alias resolution to the sandbox helper.
- Filter effective permission entries once and cache their resolved paths for
writable-root and read-only carveout construction.
- Return early when the policy has no effective writable entries.
GitOrigin-RevId: 305364173ca55f967c1701aad94d2374f78999d3