## Why
A sandboxed process could replace a writable directory root, changing the
authority boundary used to construct a later sandbox policy.
## What changed
- Deny unlinking or renaming directory roots granted write access by Seatbelt,
while preserving replacement and deletion behavior for writable files.
- Reject writable roots with nested symlink components and report these as
Seatbelt preparation errors instead of network proxy failures.
- Normalize writable roots before adding them to the generated Seatbelt policy.
## Testing
Add coverage for symlink rejection, error classification, directory-root
replacement and rename protection, newly created roots, and writable file
replacement.
GitOrigin-RevId: ab1ed4e55f20034bc43e028e6529d3d1f0d8181c
f6950546e5