fix(normalizer): don't coerce boolean cells to 1/0

Add bool guard before the int branch in _cell_to_str so True/False
cells are preserved as "True"/"False" instead of "1"/"0". Add two
regression tests covering the fix and missing-sheet error.

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

tools/import-normalizer/ingest.py

@@ -7,6 +7,8 @@ import openpyxl
 def _cell_to_str(value) -> str:
     if value is None:
         return ""
+    if isinstance(value, bool):   # bool is a subclass of int — handle before the int branch
+        return str(value)
     if isinstance(value, datetime.datetime):
         return value.date().isoformat()
     if isinstance(value, datetime.date):