Migrate snake example to observation_function and refresh its training run

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.

runs/snake/config.toml

@@ -0,0 +1,53 @@
+[game]
+module = "retro.examples.snake"
+
+[metadata]
+actions = [
+    "KEY_RIGHT",
+    "KEY_UP",
+    "KEY_LEFT",
+    "KEY_DOWN",
+]
+reward = "reward"
+extras_size = 2
+board_size = [
+    17,
+    17,
+]
+character_set = [
+    "@",
+    "*",
+    ">",
+    "<",
+    "^",
+    "v",
+]
+observation_function = "snake_observation:egocentric_observation"
+
+[preprocessing]
+spatial = false
+board = true
+observe_state = []
+
+[model]
+hidden_sizes = [
+    128,
+    64,
+]
+
+[training]
+learning_rate = 0.0001
+learning_rate_decay = 0.9999
+gamma = 0.99
+epsilon = 1.0
+epsilon_decay = 0.9997
+epsilon_min = 0.05
+batch_size = 64
+memory_capacity = 50000
+target_update_freq = 500
+train_every = 4
+training_episodes = 20000
+prioritize_experiences = true
+exploration_turns = 200
+unknown_character_strategy = "ignore"
+max_turns_per_episode = 2000