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/benchmark_snake_arena.py

@@ -5,8 +5,6 @@ For outcome/win-rate tournaments use the local Battlesnake CLI. This harness is
 fast enough for CI and detects move disagreements, crashes, and latency changes.
 """
 
-from __future__ import annotations
-
 import argparse
 import json
 import sys

scripts/benchmark_snakes_from_db.py

@@ -1,8 +1,5 @@
 #!/usr/bin/env python3
 """Benchmark snake move latency against sampled states from gameplay SQLite."""
-
-from __future__ import annotations
-
 import argparse
 from pathlib import Path
 import sqlite3
@@ -15,8 +12,8 @@ sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
 from server.database.benchmark_states import (
   _build_state, is_postgresql_source, load_postgresql_states,
 )
-from server.GameBoard import GameBoard
-from snakes import SnakeBuilder
+lazy from server.GameBoard import GameBoard
+lazy from snakes import SnakeBuilder
 
 def percentile(values: list[float], quantile: float) -> float:
   ordered = sorted(values)

scripts/cleanup_postgresql_gameplay.py

@@ -1,8 +1,5 @@
 #!/usr/bin/env python3
 """Delete old low-quality replay payloads while preserving game results."""
-
-from __future__ import annotations
-
 import argparse
 import asyncio
 from datetime import datetime, timedelta, timezone

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

scripts/merge_gameplay_databases.py

@@ -6,9 +6,6 @@ row IDs are regenerated, delta games are quality-rated, and all result rows are
 kept even when their replay is excluded. The base can be replaced only after
 verification and a timestamped backup.
 """
-
-from __future__ import annotations
-
 import argparse
 from collections import Counter
 from datetime import datetime, timezone

scripts/migrate_gameplay_database.py

@@ -5,9 +5,6 @@ The source is always opened read-only. The destination is written separately,
 verified, and can optionally replace the source after a timestamped backup is
 created. No in-place schema rewrite is performed.
 """
-
-from __future__ import annotations
-
 import argparse
 from datetime import datetime, timezone
 import json

scripts/migrate_sqlite_to_postgresql.py

@@ -5,9 +5,6 @@ The source is opened read-only. Rows are copied in bounded batches through
 temporary PostgreSQL tables, then inserted idempotently with ON CONFLICT. The
 script verifies source/inserted row counts and never modifies the SQLite file.
 """
-
-from __future__ import annotations
-
 import argparse
 import asyncio
 from datetime import datetime

scripts/run_seeded_snake_tournament.py

@@ -1,8 +1,5 @@
 #!/usr/bin/env python3
 """Run paired seeded games through the official local Battlesnake rules engine."""
-
-from __future__ import annotations
-
 import argparse
 import json
 import os

scripts/snake_arena_scenarios.py

@@ -1,7 +1,4 @@
 """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