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/core/template.py

@@ -1,7 +1,5 @@
-from typing import TYPE_CHECKING
 
-if TYPE_CHECKING:
-  from server.GameBoard import GameBoard
+lazy from server.GameBoard import GameBoard
 
 import random
 

snakes/engine/bitboard.py

@@ -7,9 +7,6 @@ no sets, deques, or per-cell Python objects.
 Typical 11×11 board → 121-bit integers.  Python big-int ops on these are
 extremely fast (single C-level limb operations under the hood).
 """
-
-from __future__ import annotations
-
 class BitBoard:
   """Pre-computed masks and fast spatial primitives for a fixed grid size."""
 

snakes/engine/duel.py

@@ -1,7 +1,4 @@
 """Reusable compact duel-search integration for Apex-style snakes."""
-
-from __future__ import annotations
-
 from snakes.engine.duel_search import BitboardDuelSearch
 
 class BitboardDuelMixin:

snakes/engine/duel_search.py

@@ -1,7 +1,4 @@
 """Deadline-aware simultaneous duel search using compact tuple bodies and bitboards."""
-
-from __future__ import annotations
-
 from collections.abc import Iterable
 from dataclasses import dataclass
 from time import perf_counter

snakes/engine/perimeter.py

@@ -1,7 +1,4 @@
 """Safety-gated board geometry scoring for perimeter-aware snakes."""
-
-from __future__ import annotations
-
 def _on_edge(point: tuple[int, int], width: int, height: int) -> bool:
   x, y = point
   return x in {0, width - 1} or y in {0, height - 1}

snakes/engine/spatial.py

@@ -1,7 +1,4 @@
 """Bitboard-backed spatial primitives shared by competitive snakes."""
-
-from __future__ import annotations
-
 from snakes.engine.bitboard import BitBoard
 
 class BitboardSpatialMixin:

snakes/engine/survival.py

@@ -1,7 +1,4 @@
 """Reusable bitboard and adversarial survival rollouts."""
-
-from __future__ import annotations
-
 from time import perf_counter
 
 from snakes.engine.survival_search import CompactSurvivalSearch

snakes/engine/survival_search.py

@@ -1,7 +1,4 @@
 """Compact adversarial rollout for multiplayer Battlesnake positions."""
-
-from __future__ import annotations
-
 from itertools import product
 from time import perf_counter
 

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
 

snakes/strategies/prism.py

@@ -32,9 +32,6 @@ Key speedups:
   S18: Prism uses a deeper tactical horizon while retaining Apex's timeout reserve.
   S19: Rollout occupancy and evaluation caches avoid repeated flood-fill work.
 """
-
-from __future__ import annotations
-
 from server.GameBoard import GameBoard
 from snakes.engine.bitboard import BitBoard
 from snakes.engine.duel import BitboardDuelMixin