Function encode_file 

pub fn encode_file<P: AsRef<Path>, Q: AsRef<Path>>(
    input_path: P,
    output_path: Q,
    max_order: Option<usize>,
) -> PpmResult<()>

Compress the file at input_path into output_path using PPM.

• max_order = None ⇒ uses the crate’s DEFAULT_ORDER (5).
• max_order = Some(n) ⇒ uses order n (up to 16).

By default, we first write an 8-byte little-endian prefix giving the original file length, then the PPM‐encoded payload.

Errors

Returns an error if any I/O or encoding step fails.