Function encode_with_lookup 

pub fn encode_with_lookup<F>(
    decomp: &DecompRef<'_>,
    lookup: F,
    out: &mut [u8; 4],
) -> Result<usize, EncodeError>
where
    F: Fn(char) -> Option<u8>,

Encode a decomposition using the supplied 字根 lookup. Writes the code bytes into out and returns the number of bytes written (3 or 4).

The encoder is the ONLY entry point that runs the rules; all platform glue (PHF runtime tables, HashMap build-time tables) goes through here via the lookup closure.