Function encode_file_meta_header 

pub fn encode_file_meta_header(meta: &FileMeta) -> String

Encode FileMeta as base64 for the crate::HEADER_FILE_META header.

The raw JSON is base64-encoded so that non-Latin-1 virtual paths (e.g. CJK filenames) survive HTTP header transport — browsers reject header values containing characters outside ISO-8859-1.