## Why
The elevated Windows sandbox requires effective `:root` read access and cannot safely enforce policies that deny reads at the filesystem root.
## What changed
- Validate root read access before elevated setup, service provisioning, and setup refresh, rejecting root-denying globs before expansion.
- Reject filesystem-root deny-read ACL targets after canonicalization and check the opened handle before applying an ACL, so aliases cannot bypass the root guard.
- Read and update deny ACLs through the same handle, and allow an existing deny ACE to be recognized when write access to the ACL is unavailable.
## Testing
Add regression coverage for default root denial, root-denying globs, allowed root reads with narrower deny rules, handle-based root detection, and reading existing deny ACEs without `WRITE_DAC`.
GitOrigin-RevId: b904e7cdbc09ee0147e1818c898b0382b5820c1b
iceweasel-oai
f11d0dd012