feat(snake): modularize engine and add tournament tools

- Split active strategies, reusable engine code, core classes, and legacy snakes.
- Replace implicit snake imports with explicit module registrations.
- Extract Prism duel, spatial, and survival behavior into focused mixins.
- Improve duel scoring with food races, pressure, caches, and depth metrics.
- Add deterministic arena scenarios and paired seeded engine tournaments.
- Expand benchmark telemetry and bump Prism to version 1.3.0.
- Update documentation and tests for the new package layout and tooling.

snakes/engine/__init__.py

@@ -0,0 +1,18 @@
+"""Reusable high-performance board and search engines for competitive snakes."""
+
+from snakes.engine.bitboard import BitBoard
+from snakes.engine.duel import BitboardDuelMixin
+from snakes.engine.duel_search import BitboardDuelSearch, DuelState
+from snakes.engine.spatial import BitboardSpatialMixin
+from snakes.engine.survival import BitboardSurvivalMixin
+from snakes.engine.survival_search import CompactSurvivalSearch
+
+__all__ = (
+  "BitBoard",
+  "BitboardDuelMixin",
+  "BitboardDuelSearch",
+  "BitboardSpatialMixin",
+  "BitboardSurvivalMixin",
+  "CompactSurvivalSearch",
+  "DuelState",
+)