Function detect_root 

pub fn detect_root(fs: &dyn FileSystemService, start: &Path) -> WorkspaceRoot

Walk up from start looking for a project or VCS marker.

The nearest ancestor holding a marker wins, so opening myrepo/src/deep/ lands on myrepo. If nothing matches all the way up, we fall back to start itself with detected: false — opening a bare directory is legitimate, not an error.