The egocentric/egocentric_player/egocentric_radius flags were removed
from retro_gamer in favor of an explicit observation_function.
snake_observation.py reproduces the old egocentric+board+extras
behavior by calling egocentric_board/encode_board/encode_state
directly, and runs/snake/config.toml now points at it.
That config change made the prior 12,000-episode checkpoint history
incompatible (retro_gamer's checkpoint compatibility checker can't
verify a new observation_function is behaviorally equivalent to the
old flags, so it conservatively refuses to resume), so the old
checkpoints were deleted and a fresh 20,000-episode run was recorded.
Track only the four checkpoints the lab actually references
(ep_1300, ep_2300, ep_4000, ep_20000) instead of all 200, and update
.gitignore so future student runs of runs/snake aren't committed by
default. snake_training.md's Q5 training-curve table and Q6 checkpoint
episodes are updated to match the real numbers from this run.
