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.

README.md

@@ -48,7 +48,7 @@ battlesnake play -W 11 -H 11 --name 'Python Starter Project' --url http://localh
 Continue with the [Battlesnake Quickstart Guide](https://docs.battlesnake.com/quickstart) to customize and improve your Battlesnake's behavior.
 
 ## Included Competitive Snake
-This repo now includes `snakes/BestBattleSnake.py`, a stronger default snake that combines:
+This repo retains `snakes/legacy/BestBattleSnake.py`, a stronger historical snake that combines:
 - collision and head-to-head risk checks
 - flood-fill space evaluation to avoid traps
 - food routing that gets more aggressive as health drops
@@ -66,12 +66,23 @@ BATTLE_SNAKE_DUEL_STYLE=balanced python main.py
 
 Allowed values: `safe`, `balanced`, `aggressive`.
 
+## Snake package layout
+The snake code is split by responsibility:
+- `snakes/strategies/` — actively maintained Apex and Prism entry points
+- `snakes/engine/` — reusable bitboards, spatial mixins, duel search, and survival search
+- `snakes/core/` — shared base classes
+- `snakes/legacy/` — historical snakes retained for compatibility and benchmarks
+
+Snake selection still uses the existing registry names, so deployment values such
+as `SNAKE=PrismBattleSnake_GPT_5_6_Sol` remain unchanged.
+
 ## PrismBattleSnake_GPT_5_6_Sol
 `PrismBattleSnake_GPT_5_6_Sol` is a separate snake that keeps Apex's strategy while
 accelerating hot spatial operations with a Python-integer bitboard engine. It
 also shares duel transpositions across candidate moves, uses principal-variation
-ordering and aspiration windows, and runs a compact adversarial multiplayer
-rollout with simultaneous enemy responses. Its filename, class, and registry
+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
 key include the model name, while its public Battlesnake API name remains
 `PrismBattleSnake`.
 
@@ -95,11 +106,24 @@ Run the deterministic CI-friendly arena benchmark without a gameplay database:
 just bench-snake-arena positions=100
 ```
 
-It reports latency, reached minimax depth, and move disagreements between Apex
-and Prism. Add `output=data/arena-report.json` to save a machine-readable report.
+It rotates through duel, hazard, multiplayer, constrictor, and cramped-endgame
+positions. It reports latency, completed duel/rollout depth, searched nodes,
+cache hits, deadline exits, and move disagreements between Apex and Prism. Use
+`--scenario hazard` (repeatable) when invoking the Python script to isolate a
+scenario. Add `output=data/arena-report.json` to save a machine-readable report.
 For representative strategy evaluation, provide recorded positions to
-`scripts/benchmark_snake_arena.py --database /path/to/gameplay.sqlite3`, then run
-paired seeded games with the local Battlesnake CLI to measure win rate.
+`scripts/benchmark_snake_arena.py --database /path/to/gameplay.sqlite3`.
+
+Run paired seeded games through the official local Battlesnake rules engine:
+```sh
+just bench-snake-tournament games=20 gametype=standard map=standard
+```
+
+Each seed is played twice with Apex and Prism swapping initial engine slots. The
+report includes wins, draws, win rates, and average game length. Save all
+per-game results with `output=data/tournament-report.json`. The tournament starts
+both snake servers with gameplay persistence disabled, adds the engine identity
+header required by the API, and shuts them down when finished.
 
 ## Compact gameplay database
 New gameplay turns use normalized storage: the turn row stores food, hazards,