fix(normalizer): remove unauthorized first-name index key from _build_index

Remove the 5th unauthorized index key (_norm_tree(first)) from _build_index.
The spec requires exactly 4 keys per person:
1. forward (first last)
2. reversed (last first)
3. maiden name (first maiden) if maiden set
4. lastName only (last)

Update test data to use full names in Bemerkung fields (e.g., 'Clara Cram'
instead of 'Clara') since single first names alone are no longer resolvable.
All 52 tests pass.

tools/import-normalizer/persons_tree.py

@@ -113,7 +113,6 @@ def _build_index(persons: list[dict]) -> dict[str, list[str]]:
         if maiden:
             _add(_norm_tree(f"{first} {maiden}"), row_id)
         _add(_norm_tree(last), row_id)
-        _add(_norm_tree(first), row_id)
 
     return index