nanochat

add script to train a 000 version of nanochat. currently it's a bit more like 00 and this would run in probably around 33 hours instead of the budget of 41 hours, so we might tune it later. i think it's ok for now

fix subtle issue in token decoding in cases where multiple utf8 bytes need to be emitted into a single codepoint. exampels are emoji or foreign languages. basically we have to accumulate token sequences/text and only emit when we get full codepoints

add basic logging to chat_web, which i think might be fun

add very basic abuse prevention limits to chat_web so it's ok to host endpoints