feat: auto-checkpoint on SIGTERM/SIGINT with DDP support

- Register SIGTERM/SIGINT handlers in training loop, set stop flag on signal
- Check stop_requested at each epoch/batch boundary, break and call on_error to save checkpoint
- LocalStrategy parent forwards signal to child processes via terminate(), waits up to 600s for graceful exit
- TrainContext gains threading.Event-based stop_requested/request_stop
- Tests verify SIGTERM/SIGINT trigger checkpoint save with exit code 0, works on both CPU and GPU

astrai/trainer/train_context.py

@@ -1,3 +1,4 @@
+import threading
 from dataclasses import dataclass, field
 from pathlib import Path
 from typing import Any, Dict, Optional, Self
@@ -41,6 +42,15 @@ class TrainContext:
     rank: int = field(default=0)
     kwargs: Dict[str, Any] = field(default_factory=dict)
 
+    _stop_event: threading.Event = field(default_factory=threading.Event)
+
+    @property
+    def stop_requested(self) -> bool:
+        return self._stop_event.is_set()
+
+    def request_stop(self) -> None:
+        self._stop_event.set()
+
     @property
     def optimizer_step(self) -> int:
         return self.consumed_samples // (