The out-of-project read guard treated any non-absolute path as cwd-relative, but on
Windows a rooted-but-driveless path (`\foo\bar`) is not absolute yet resolves against
the current drive root, outside the project. Classify with a platform-correct predicate
(`not root and not drive` under the host path flavour) so the containment check is
reached. On POSIX the predicate reduces to `not is_absolute()`, so no behaviour changes
there.
