Function update_fxar_ai_sections 

pub fn update_fxar_ai_sections(
    path: &Path,
    sections: &[(u32, Vec<u8>)],
) -> Result<()>

Update the AI extension sections of an existing FXAR archive in-place.

Only the extension-section tail is rewritten – manifest, chunk index, and chunk data are never touched. Handles both archives that already have extension sections (update) and those that don’t (append).

sections is a list of (section_type, raw_bytes) pairs, matching the format used by write_archive_seekable_with_ai.