Function prepare 

pub fn prepare(
    items: &[Resolved],
    missing: &[String],
    inline_max_count: usize,
    inline_budget: u64,
) -> Prepared

Build the list and (when inline_max_count > 0) the inline blocks, using at most inline_budget base64 bytes (further capped at INLINE_MAX_B64_BYTES per message). Reads the inlined files from disk. Blocking.