Function fetch_path 

pub fn fetch_path(
    path: &Path,
    project_root: &Path,
    source_name: &str,
) -> Result<ResolvedRef, MarsError>

Fetch a path source: resolve relative paths against project root, verify exists.

• Relative paths are resolved against project_root.
• Absolute paths are used as-is.
• The path must exist and be a directory.
• Returns ResolvedRef with no version/commit (path sources are unversioned).