Sandbox is a no-op when root is at or above `$HOME`

A root at or above $HOME silently disables confinement.

The profile ends with:

(deny file* (subpath user_home))
(allow file-read-metadata (subpath user_home))
(allow file* (subpath sandbox_root))

SBPL is last-match-wins, so the sandbox_root allow has to come after the user_home deny for the normal case (a project inside $HOME) to work. But that also means a root equal to or above $HOME re-allows all of $HOME.

Repro: run with DUNE_ROOT="$HOME" (or any ancestor, e.g. /Users) and the sandboxed process has full read/write of the home directory. No warning is printed, the sandbox applies successfully and confines nothing.

Reordering the rules is not a fix: putting the root allow first makes the home deny win for every project under $HOME, breaking the common case. The fix is to reject such a root. sandboxer already validates that --root and --home are absolute and are directories, so a containment check could fit alongside those?

Start at the sandboxer entry point and read the existing --root and --home validation for absolute paths and directories. Add the missing rejection for roots equal to or above the home directory, then verify that a project below $HOME still remains confined and invalid roots fail before the sandbox is applied.