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.

scripts/download_snake_customizations.py

@@ -1,10 +1,7 @@
 #!/usr/bin/env python3
-
-from __future__ import annotations
-
 import argparse
 from pathlib import Path
-from typing import Iterable
+from collections.abc import Iterable
 from urllib.parse import urlencode
 from urllib.request import urlopen
 import shutil
@@ -84,14 +81,14 @@ def prune_output(output_root:Path, wanted_files:set[Path]) -> int:
     return 0
 
   for file_path in output_root.rglob("*"):
-    if not file_path.is_file():
+    if not file_path.info.is_file():
       continue
     if file_path not in wanted_files:
       file_path.unlink()
       removed += 1
 
   for directory in sorted(
-    (p for p in output_root.rglob("*") if p.is_dir()), reverse=True
+    (p for p in output_root.rglob("*") if p.info.is_dir()), reverse=True
   ):
     if any(directory.iterdir()):
       continue