클래식 공개 API 정리

맥락:
- GUI 이식 전에 classic 패키지 루트에서 노출할 API 범위를 정리했다.
- Claude Opus 4.7 xhigh 자문에서 root export 축소와 backend builder 이름 단일화를 우선 권장했다.

변경:
- classic package root export를 GameState, config, env, backend, bot registry 중심으로 줄였다.
- backend factory 이름을 build_backend로 단일화하고 README 사용 예시를 갱신했다.
- public API smoke test를 추가하고 pyproject description 및 Cython build failure 처리를 정리했다.

확인:
- uv run pytest tests/games/classic
- uv run lost-cities-classic

setup.py

@@ -4,27 +4,25 @@ from setuptools import Extension, setup
 
 try:
     from Cython.Build import cythonize
-except ImportError:  # pragma: no cover
-    cythonize = None
+except ImportError as exc:  # pragma: no cover
+    raise RuntimeError("Cython is required to build coolrl-lost-cities") from exc
 
 
-extensions: list[Extension] = []
-if cythonize is not None:
-    extensions = cythonize(
-        [
-            Extension(
-                "coolrl_lost_cities.games.classic.game",
-                ["src/coolrl_lost_cities/games/classic/game.pyx"],
-            )
-        ],
-        language_level=3,
-        compiler_directives={
-            "boundscheck": False,
-            "wraparound": False,
-            "cdivision": True,
-            "initializedcheck": False,
-        },
-    )
+extensions = cythonize(
+    [
+        Extension(
+            "coolrl_lost_cities.games.classic.game",
+            ["src/coolrl_lost_cities/games/classic/game.pyx"],
+        )
+    ],
+    language_level=3,
+    compiler_directives={
+        "boundscheck": False,
+        "wraparound": False,
+        "cdivision": True,
+        "initializedcheck": False,
+    },
+)
 
 
 setup(ext_modules=extensions)