security: add atomic write utilities for TOCTOU protection (V-015)

Add atomic_write.py with temp file + rename pattern to prevent
Time-of-Check to Time-of-Use race conditions in file operations.

CVSS: 7.4 (High)
Refs: V-015
CWE-367: TOCTOU Race Condition

tools/atomic_write.py

@@ -0,0 +1,64 @@
+"""Atomic file write operations to prevent TOCTOU race conditions.
+
+SECURITY FIX (V-015): Implements atomic writes using temp files + rename
+to prevent Time-of-Check to Time-of-Use race conditions.
+
+CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition
+"""
+
+import os
+import tempfile
+from pathlib import Path
+from typing import Union
+
+
+def atomic_write(path: Union[str, Path], content: str, mode: str = "w") -> None:
+    """Atomically write content to file using temp file + rename.
+    
+    This prevents TOCTOU race conditions where the file could be
+    modified between checking permissions and writing.
+    
+    Args:
+        path: Target file path
+        content: Content to write
+        mode: Write mode ("w" for text, "wb" for bytes)
+    """
+    path = Path(path)
+    path.parent.mkdir(parents=True, exist_ok=True)
+    
+    # Write to temp file in same directory (same filesystem for atomic rename)
+    fd, temp_path = tempfile.mkstemp(
+        dir=path.parent,
+        prefix=f".tmp_{path.name}.",
+        suffix=".tmp"
+    )
+    
+    try:
+        if "b" in mode:
+            os.write(fd, content if isinstance(content, bytes) else content.encode())
+        else:
+            os.write(fd, content.encode() if isinstance(content, str) else content)
+        os.fsync(fd)  # Ensure data is written to disk
+    finally:
+        os.close(fd)
+    
+    # Atomic rename - this is guaranteed to be atomic on POSIX
+    os.replace(temp_path, path)
+
+
+def safe_read_write(path: Union[str, Path], content: str) -> dict:
+    """Safely read and write file with TOCTOU protection.
+    
+    Returns:
+        dict with status and error message if any
+    """
+    try:
+        # SECURITY: Use atomic write to prevent race conditions
+        atomic_write(path, content)
+        return {"success": True, "error": None}
+    except PermissionError as e:
+        return {"success": False, "error": f"Permission denied: {e}"}
+    except OSError as e:
+        return {"success": False, "error": f"OS error: {e}"}
+    except Exception as e:
+        return {"success": False, "error": f"Unexpected error: {e}"}