migration: close candidate maintenance audit

app/tools/split_frontend_runtime.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import argparse
 import hashlib
 import json
 import re
@@ -91,6 +92,20 @@ def extract_written_chunks(paths: list[Path]) -> dict[tuple[int, int], str]:
 
 
 def main() -> None:
+    parser = argparse.ArgumentParser(
+        description="Reproduce the one-time Slice 10 frontend source split"
+    )
+    parser.add_argument(
+        "--apply",
+        action="store_true",
+        help="perform the historical split; this rewrites frontend runtime files",
+    )
+    args = parser.parse_args()
+    if not args.apply:
+        parser.error(
+            "this migration-only tool rewrites files; pass --apply only when "
+            "reproducing Slice 10 from its documented pre-split checkpoint"
+        )
     source_path = ORIGINAL_STATIC / "app.js"
     target_path = FRONTEND_ROOT / "app.js"
     source_bytes = source_path.read_bytes()