feat!: adopt Python 3.15 and support BattlesnakeArena

- Require Python 3.15 in project metadata, runtime pin, and Docker stages.
- Use lazy imports for database backends and type-only dependencies.
- Defer benchmark dependencies until their code paths are used.
- Rely on deferred annotations and use Path.info for file discovery.
- Accept BattlesnakeArena alongside BattlesnakeEngine on game endpoints.
- Advance quart_common for Python 3.15 and multi-agent validation.
- Add pytest development dependencies and refresh the uv lockfile.
- Test lazy backend loading, import order, and fresh file discovery.
- Document the runtime requirements and locked uv workflow.

BREAKING CHANGE: Python 3.15 or newer is required; earlier runtimes cannot parse the lazy import syntax.

snakes/legacy/SupremeBattleSnake_ClaudeOpus4_6.py

@@ -24,8 +24,6 @@ Key speedups:
        per-direction Python loop + _in_bounds calls.
   S11: _future_survival_tree inlines legal-move check with bitboard ops.
 """
-
-from __future__ import annotations
 from typing import Any
 from time import perf_counter