feat(question_gen): QuestionGenerator Protocol + 模块公开 API

app/ports.py 追加 QuestionGenerator Protocol（预留 LLM 出题接口）。
app/question_gen/__init__.py re-export load_benchmark 和 stratified_sample。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/unit/test_question_gen_api.py

@@ -0,0 +1,40 @@
+"""app/ports.py QuestionGenerator Protocol 与 app/question_gen 公开 API 测试。"""
+
+from __future__ import annotations
+
+import importlib
+
+from app.ports import QuestionGenerator
+
+
+class TestQuestionGeneratorProtocol:
+    def test_importable(self) -> None:
+        """QuestionGenerator 可从 app.ports 导入。"""
+        assert QuestionGenerator is not None
+
+    def test_is_runtime_checkable(self) -> None:
+        """QuestionGenerator 是 runtime_checkable Protocol。"""
+        assert hasattr(QuestionGenerator, "__protocol_attrs__") or hasattr(
+            QuestionGenerator, "__abstractmethods__"
+        )
+
+    def test_generate_method_exists(self) -> None:
+        """Protocol 定义了 generate 方法。"""
+        assert hasattr(QuestionGenerator, "generate")
+
+
+class TestQuestionGenPublicAPI:
+    def test_load_benchmark_importable_from_package(self) -> None:
+        """load_benchmark 可从 app.question_gen 直接导入。"""
+        mod = importlib.import_module("app.question_gen")
+        assert hasattr(mod, "load_benchmark")
+
+    def test_stratified_sample_importable_from_package(self) -> None:
+        """stratified_sample 可从 app.question_gen 直接导入。"""
+        mod = importlib.import_module("app.question_gen")
+        assert hasattr(mod, "stratified_sample")
+
+    def test_all_exports(self) -> None:
+        """__all__ 包含预期的公开 API。"""
+        mod = importlib.import_module("app.question_gen")
+        assert set(mod.__all__) == {"load_benchmark", "stratified_sample"}