refactor: extract atomic_json_write helper, add 24 checkpoint tests

Extract the duplicated temp-file + fsync + os.replace pattern from
batch_runner.py (1 instance) and process_registry.py (2 instances) into
a shared utils.atomic_json_write() function.

Add 12 tests for atomic_json_write covering: valid JSON, parent dir
creation, overwrite, crash safety (original preserved on error), no temp
file leaks, string paths, unicode, custom indent, concurrent writes.

Add 12 tests for batch_runner checkpoint behavior covering:
_save_checkpoint (valid JSON, last_updated, overwrite, lock/no-lock,
parent dirs, no temp leaks), _load_checkpoint (missing file, existing
data, corrupt JSON), and resume logic (preserves prior progress,
different run_name starts fresh).

batch_runner.py

@@ -29,8 +29,6 @@ from typing import List, Dict, Any, Optional, Tuple
 from datetime import datetime
 from multiprocessing import Pool, Lock
 import traceback
-import tempfile
-
 from rich.progress import Progress, SpinnerColumn, BarColumn, TextColumn, TimeRemainingColumn, MofNCompleteColumn
 from rich.console import Console
 import fire
@@ -703,32 +701,12 @@ class BatchRunner:
         """
         checkpoint_data["last_updated"] = datetime.now().isoformat()
 
-        def _atomic_write():
-            """Write checkpoint atomically (temp file + replace) to avoid corruption on crash."""
-            self.checkpoint_file.parent.mkdir(parents=True, exist_ok=True)
-            fd, tmp_path = tempfile.mkstemp(
-                dir=str(self.checkpoint_file.parent),
-                prefix='.checkpoint_',
-                suffix='.tmp',
-            )
-            try:
-                with os.fdopen(fd, 'w', encoding='utf-8') as f:
-                    json.dump(checkpoint_data, f, indent=2, ensure_ascii=False)
-                    f.flush()
-                    os.fsync(f.fileno())
-                os.replace(tmp_path, self.checkpoint_file)
-            except BaseException:
-                try:
-                    os.unlink(tmp_path)
-                except OSError:
-                    pass
-                raise
-
+        from utils import atomic_json_write
         if lock:
             with lock:
-                _atomic_write()
+                atomic_json_write(self.checkpoint_file, checkpoint_data)
         else:
-            _atomic_write()
+            atomic_json_write(self.checkpoint_file, checkpoint_data)
     
     def _scan_completed_prompts_by_content(self) -> set:
         """