feat(products): diff engine — add/update/unchanged/error + INV-11 fingerprint

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/domains/products/validate.py

@@ -31,8 +31,10 @@ from .models import (
 
 _HANDLE_RE = re.compile(r"^[a-z0-9-]+$")
 _STATUSES = {"draft", "active", "archived"}
-# Product columns handled as attributes (title, option_names), not via fields{}.
-_ATTRIBUTE_COLUMNS = {"Title", "Option1 Name", "Option2 Name", "Option3 Name"}
+# Title is handled as an attribute, not via fields{}. Option names live in BOTH
+# .option_names (the values) and fields{} (the file-presence signal the diff
+# needs: absent column == untouched, never a clear).
+_ATTRIBUTE_COLUMNS = {"Title"}
 # Sentinel: the cell failed normalization (the error is already recorded).
 _INVALID = object()