Function find_case_insensitive_file 

pub fn find_case_insensitive_file(
    directory: &Path,
    expected_file_name: &str,
) -> Result<Option<PathBuf>>

Finds one file in directory with ASCII case-insensitive name matching.

When multiple entries differ only by case, this function returns a deterministic winner by sorting candidates on:

1. lowercase filename
2. original filename bytes