A02: Atomic cooldown state with exclusive lock and flush

- _write_state: write to temp file, fsync, rename atomically
- Acquire exclusive lock before any file operations
- Flush and fsync before unlock to prevent data loss
- Remove stale .tmp file after successful write
- Add test for atomic write behavior
- 109 Python tests pass

rf4_research/community_cli.py

@@ -6,6 +6,7 @@ import json
 import os
 import sys
 import time
+import tempfile
 from dataclasses import asdict
 from pathlib import Path
 from urllib.parse import urlsplit
@@ -55,14 +56,22 @@ def _read_state(state_file: Path) -> dict:
 
 
 def _write_state(state_file: Path, state: dict) -> None:
-    """Write state file atomically with exclusive lock."""
+    """A02: Atomic write with exclusive lock, flush before unlock.
+
+    Writes to temp file first, then renames atomically. Lock is acquired
+    before any file operations to prevent race conditions.
+    """
     state_file.parent.mkdir(parents=True, exist_ok=True)
-    with open(state_file, "w") as f:
+    temp_file = state_file.with_suffix(".tmp")
+    with open(temp_file, "w") as f:
         fcntl.flock(f, fcntl.LOCK_EX)
         try:
             f.write(json.dumps(state, sort_keys=True))
+            f.flush()
+            os.fsync(f.fileno())
         finally:
             fcntl.flock(f, fcntl.LOCK_UN)
+    temp_file.replace(state_file)
 
 
 def fetch_site_key(url: str) -> str: