Function pd_markdown_html 

#[unsafe(no_mangle)]
pub extern "C" fn pd_markdown_html(
    text_ptr: *const u8,
    text_len: u32,
) -> u64

Render markdown to safe HTML (the subset in packdiff_dto::markdown). The input is the raw markdown text — a plain UTF-8 string, not JSON — and Ok carries the HTML string. Never fails on any input.