refactor: 重构训练后端为 Executor 模式

- backend.py → executor.py，BaseTrainingBackend → BaseExecutor
- 新增 NoneExecutor（单卡）和 DDPExecutor（DDP，world_size=1 自动降级）
- 新增 GradientState 分离梯度同步状态，AccumOptimizer/AccumScheduler 包裹拦截
- 新增 astrai/protocols.py：OptimizerProtocol/SchedulerProtocol 结构子类型
- TrainContext.backend → executor，TrainConfig 移除 parallel_wrapper/state_dict_fn，新增 parallel_mode/executor_kwargs
- 训练循环用 accumulate() 包裹，on_optimizer_step 命名约定=gate
- scripts/tools/train.py 移除 ddp_wrap/prepare_checkpoint，新增 --parallel_mode

astrai/protocols.py

@@ -0,0 +1,21 @@
+"""Training component protocols — structural subtyping for optimizer/scheduler wrappers."""
+
+from typing import Any, Protocol, runtime_checkable
+
+
+@runtime_checkable
+class OptimizerProtocol(Protocol):
+    def step(self, closure=None): ...
+    def zero_grad(self): ...
+    @property
+    def param_groups(self) -> Any: ...
+    def state_dict(self) -> dict: ...
+    def load_state_dict(self, d: dict): ...
+
+
+@runtime_checkable
+class SchedulerProtocol(Protocol):
+    def step(self): ...
+    def state_dict(self) -> dict: ...
+    def load_state_dict(self, d: dict): ...
+    def get_last_lr(self): ...