perf(snake): cache survival rollout state

- Cache occupancy bitboards for repeated multiplayer rollout positions.
- Memoize position evaluations to avoid duplicate flood-fill calculations.
- Reuse shared values while ranking simultaneous enemy responses.
- Include evaluation hits in Prism rollout telemetry.
- Document the optimization and bump Prism to version 1.4.0.

README.md

@@ -82,7 +82,7 @@ accelerating hot spatial operations with a Python-integer bitboard engine. It
 also shares duel transpositions across candidate moves, uses principal-variation
 ordering, aspiration windows, path-aware food races, and a deeper tactical
 horizon, and runs a compact adversarial multiplayer rollout with simultaneous
-enemy responses. Its filename, class, and registry
+enemy responses and cached occupancy/evaluation states. Its filename, class, and registry
 key include the model name, while its public Battlesnake API name remains
 `PrismBattleSnake`.