Function try_extract_user_function 

pub(crate) fn try_extract_user_function(
    src: &str,
    start: usize,
) -> Option<(usize, String)>

Try to match a single HLSL-shaped function definition starting at byte position start (after leading whitespace). Returns the byte position just past the closing } and the rewritten WGSL function text. Returns None if no signature matches — caller advances by 1 byte.