Handle INIT messages and add set_init_handler() callback

[brocci]

Init (I) messages are not consumed by _decode(), leaving payload bytes in the serial buffer where they can corrupt the next frame.
Sketches also need a way to react to the configuration data (NDP, transmit delay, options) that JMRI sends at startup.

Goals:

1. Route INIT packets through the data decoder so the payload is properly consumed (with DLE un-escaping) instead of discarded
2. Return the actual packet type (INIT vs SET) from _decode() so the caller can distinguish them
3. Provide an optional callback via set_init_handler() so sketches can inspect NDP, delay, and option bytes without manual polling
4. Include an example showing how to use the callback