refactor: 设计模式优化 inference 模块导入结构

- 新建 cache.py：SlotAllocator 对象池 + PrefixCacheManager

- 新建 sampling.py：Temperature/TopK/TopP 可组合策略

- TaskStatus 改用 Enum，GenerationParams 值对象模式

- _STOP 移至 cache.py，解除 engine→scheduler 轻量耦合

- 更新测试导入路径，ruff 格式检查通过

astrai/inference/__init__.py

@@ -1,25 +1,46 @@
-"""Inference module for continuous batching."""
+"""Inference module for continuous batching.
+
+Layers:
+  - engine.py:    Facade (InferenceEngine), Value Object (GenerationParams, GenerationRequest)
+  - scheduler.py: Continuous-batching loop, Task state machine, TaskStatus enum
+  - cache.py:     Object Pool (SlotAllocator), PrefixCacheManager
+  - sampling.py:  Strategy pattern (TemperatureStrategy, TopKStrategy, TopPStrategy)
+  - server.py:    FastAPI HTTP server (OpenAI-compatible endpoints)
+"""
 
 from astrai.inference.engine import (
+    GenerationParams,
     GenerationRequest,
     InferenceEngine,
 )
+from astrai.inference.sampling import (
+    BaseSamplingStrategy,
+    SamplingPipeline,
+    TemperatureStrategy,
+    TopKStrategy,
+    TopPStrategy,
+    apply_sampling_strategies,
+)
 from astrai.inference.scheduler import (
     InferenceScheduler,
     Task,
     TaskStatus,
-    apply_sampling_strategies,
 )
 
 __all__ = [
-    # Engine
+    # Engine / Requests
     "InferenceEngine",
+    "GenerationRequest",
+    "GenerationParams",
     # Scheduler
     "InferenceScheduler",
     "Task",
     "TaskStatus",
-    # Request
-    "GenerationRequest",
-    # Sampling
+    # Sampling (Strategy pattern)
     "apply_sampling_strategies",
+    "BaseSamplingStrategy",
+    "TemperatureStrategy",
+    "TopKStrategy",
+    "TopPStrategy",
+    "SamplingPipeline",
 ]