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.

justfile

@@ -70,6 +70,14 @@ bench-snake-arena positions="100" output="":
   if [ -n "{{output}}" ]; then args+=(--json-output "{{output}}"); fi
   PYTHONPATH="{{justfile_directory()}}" python "{{justfile_directory()}}/scripts/benchmark_snake_arena.py" "${args[@]}"
 
+bench-snake-tournament games="20" gametype="standard" map="standard" output="":
+  #!/usr/bin/env bash
+  set -euo pipefail
+
+  args=(--games "{{games}}" --gametype "{{gametype}}" --map "{{map}}")
+  if [ -n "{{output}}" ]; then args+=(--json-output "{{output}}"); fi
+  PYTHONPATH="{{justfile_directory()}}" python "{{justfile_directory()}}/scripts/run_seeded_snake_tournament.py" "${args[@]}"
+
 build-battlesnake-cli:
   #!/usr/bin/env bash
   set -euo pipefail