fix(normalizer): defang leading LF in CSV + assert pinned workbook timestamp

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tools/import-normalizer/writers.py

@@ -19,7 +19,7 @@ def _join(value):
 def _csv_safe(value):
     """Neutralise spreadsheet formula injection (CWE-1236) in human-opened review CSVs."""
     s = "" if value is None else str(value)
-    return "'" + s if s[:1] in ("=", "+", "-", "@", "\t", "\r") else s
+    return "'" + s if s[:1] in ("=", "+", "-", "@", "\t", "\r", "\n") else s
 
 
 DOC_COLUMNS = ["index", "box", "folder", "sender_person_id", "sender_name",