Function apply_chunking 

pub fn apply_chunking(
    markdown: &str,
    budget: usize,
    chunk: Option<usize>,
    extra_fm: &[(&str, &str)],
) -> Result<String, String>

Apply chunking to a markdown response. Returns the chunk at idx (0-based) with appropriate frontmatter metadata.

_chunk: N and _total_chunks: M are always injected (including the 1 of 1 case) so an agent walking the surface can size pagination without first peeking at the response length. A request for chunk > total_chunks is always an error — even when the body fits in a single chunk and the silent-cap behaviour would have hidden the overshoot.