fix: publish checkpoints atomically

- Write checkpoint payloads to a hidden sibling staging directory, add a versioned checksum manifest, fsync the completed payload, and publish it with an atomic rename
- Republishing an existing step retires the old payload under a hidden sibling name before the atomic rename, so re-runs into the same output directory replace the previous checkpoint instead of raising FileExistsError
- Keep legacy checkpoints loadable, add optional checksum verification, and align metric flushing with checkpoint publication

Co-authored-by: 0z5a <dezhen.lu@student.uni-tuebingen.de>

docs/developer/architecture.md

@@ -259,8 +259,8 @@ classDiagram
             +dict meta
             +dict config
             +save(save_dir)
-            +load(save_dir, broadcast) Checkpoint
-            +load_any(save_dir, broadcast) Optional[Checkpoint]
+            +load(save_dir, broadcast, verify_checksums) Checkpoint
+            +load_any(save_dir, broadcast, verify_checksums) Optional[Checkpoint]
         }
     }