feat: add --resume flag to decouple weight loading from training resumption

- Add --resume bool flag to train.py CLI
- --param_path always loads weights only by default
- --resume restores epoch, consumed_samples, optimizer & scheduler
- Checkpoint.load() now preserves full meta dict
- Update test_early_stopping to use new param_path/resume API

astrai/serialization/checkpoint.py

@@ -2,7 +2,6 @@
 
 import io
 import json
-import os
 import time
 from dataclasses import dataclass, field
 from pathlib import Path
@@ -178,6 +177,7 @@ class Checkpoint:
             epoch=meta.get("epoch", 0),
             consumed_samples=meta.get("consumed_samples", 0),
             extra=extra,
+            meta=meta,
             config=config,
         )