refactor: TaskManager 剥离页管理，STOP 移至 task.py

- TaskManager 移除 page_cache/page_size 依赖，增 pull_candidates/activate/return_to_waiting
- Executor 增 allocate_pages_for_activation/free_task_pages，承接全部页操作
- STOP 从 cache.py 移至 task.py
- scheduler loop 显式装配: 清理→释页 / 拉取→分配→激活
- sampling.py → sample.py

astrai/inference/engine.py

@@ -16,8 +16,8 @@ from typing import Any, AsyncGenerator, Dict, Generator, List, Optional, Tuple,
 import torch
 import torch.nn as nn
 
-from astrai.inference.cache import STOP
 from astrai.inference.scheduler import InferenceScheduler
+from astrai.inference.task import STOP
 from astrai.tokenize import AutoTokenizer