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.

scripts/snake_arena_scenarios.py

@@ -0,0 +1,108 @@
+"""Deterministic scenario corpus for the local snake arena."""
+
+from __future__ import annotations
+
+from copy import deepcopy
+
+from tests.bench_best_battle_snake import build_game_state
+
+def _state(payload: dict, scenario: str, index: int) -> tuple[dict, dict]:
+  payload["game"]["id"] = f"arena-{scenario}-{index}"
+  return payload["board"], {
+    "game_id": payload["game"]["id"],
+    "source": "custom",
+    "map": payload["game"].get("map", "standard"),
+    "ruleset": payload["game"]["ruleset"],
+    "turn": payload["turn"],
+    "you": payload["you"],
+    "scenario": scenario,
+  }
+
+def _standard_duel(index: int) -> dict:
+  payload = build_game_state()
+  payload["turn"] = 20 + index
+  payload["board"]["food"] = [
+    {"x": 1 + index % 3, "y": 9},
+    {"x": 9, "y": 1 + (index // 3) % 3},
+  ]
+  return payload
+
+def _hazard_duel(index: int) -> dict:
+  payload = _standard_duel(index)
+  payload["you"]["health"] = 38 + index % 12
+  payload["board"]["snakes"][0]["health"] = payload["you"]["health"]
+  hazard_x = 5 + index % 2
+  payload["board"]["hazards"] = [
+    {"x": hazard_x, "y": y} for y in range(1, 10) if y != 5
+  ]
+  return payload
+
+def _multiplayer(index: int) -> dict:
+  payload = _standard_duel(index)
+  third = {
+    "id": "enemy-2",
+    "name": "enemy-2",
+    "health": 65,
+    "length": 5,
+    "head": {"x": 2, "y": 8},
+    "body": [
+      {"x": 2, "y": 8}, {"x": 2, "y": 9}, {"x": 2, "y": 10},
+      {"x": 1, "y": 10}, {"x": 0, "y": 10},
+    ],
+  }
+  payload["board"]["snakes"].append(third)
+  return payload
+
+def _constrictor(index: int) -> dict:
+  payload = _multiplayer(index)
+  payload["game"]["ruleset"] = deepcopy(payload["game"]["ruleset"])
+  payload["game"]["ruleset"]["name"] = "constrictor"
+  payload["board"]["food"] = []
+  return payload
+
+def _cramped_duel(index: int) -> dict:
+  payload = _standard_duel(index)
+  payload["board"]["width"] = 7
+  payload["board"]["height"] = 7
+  mine = {
+    "id": "me", "name": "me", "health": 72, "length": 7,
+    "head": {"x": 2, "y": 3},
+    "body": [
+      {"x": 2, "y": 3}, {"x": 2, "y": 2}, {"x": 2, "y": 1},
+      {"x": 1, "y": 1}, {"x": 1, "y": 2}, {"x": 1, "y": 3},
+      {"x": 1, "y": 4},
+    ],
+  }
+  enemy = {
+    "id": "enemy", "name": "enemy", "health": 72, "length": 7,
+    "head": {"x": 4, "y": 3},
+    "body": [
+      {"x": 4, "y": 3}, {"x": 4, "y": 2}, {"x": 4, "y": 1},
+      {"x": 5, "y": 1}, {"x": 5, "y": 2}, {"x": 5, "y": 3},
+      {"x": 5, "y": 4},
+    ],
+  }
+  payload["you"] = mine
+  payload["board"]["snakes"] = [mine, enemy]
+  payload["board"]["food"] = [{"x": 3, "y": 5 + index % 2}]
+  payload["board"]["hazards"] = []
+  return payload
+
+SCENARIOS = {
+  "duel": _standard_duel,
+  "hazard": _hazard_duel,
+  "multi": _multiplayer,
+  "constrictor": _constrictor,
+  "cramped": _cramped_duel,
+}
+
+def synthetic_states(count: int, scenarios: list[str] | None = None) -> list[tuple[dict, dict]]:
+  selected = scenarios or list(SCENARIOS)
+  unknown = set(selected) - set(SCENARIOS)
+  if unknown:
+    raise ValueError(f"Unknown arena scenarios: {', '.join(sorted(unknown))}")
+  states: list[tuple[dict, dict]] = []
+  for index in range(count):
+    scenario = selected[index % len(selected)]
+    states.append(_state(SCENARIOS[scenario](index), scenario, index))
+  return states