Data review: document the historical GeoNames province code SU

[Codewriter90x]

The deterministic reconciliation backlog identifies SU as a GeoNames source province code that is not present among the current ISTAT municipality province codes. It occurs only on unreconciled source localities and is not promoted to current ISTAT classification.

Required evidence:

• authoritative or appropriately licensed mapping/history for the code;
• reference dates and attribution;
• affected source-record count and region/province segmentation;
• a reproducible alias or migration rule, if justified;
• proof that no ambiguous locality is silently promoted.

Public Nominatim and manual edits to generated exports are out of scope.