Function header_cache_path 

pub fn header_cache_path(repo_dir: &Path, filename: &str, etag: &str) -> PathBuf

Header-cache entry path: {repo_dir}/.hf-fm-header-cache/{sanitized filename}.{etag}.json.

filename may contain path separators (a nested file, e.g. subdir/model.gguf) — sanitized to a single path component by replacing / and \ with __, so the cache directory never needs subdirectories of its own. Uses string concatenation rather than Path::with_extension so an etag containing periods round-trips correctly, the same rationale as temp_blob_path / temp_state_path.