total_tax_add() improvements

Case handling (matching): Current code lowercases the key column but
not tax_names; documented “not case sensitive” is therefore incorrect.
Normalize both sides internally (e.g., to_lower() on inputs).

Case handling (new row search): The NA→0 household fix searches the
lower-cased key column against the raw total_tax_name. With the
default "TOTAL_TAX" this may fail. Normalize in both places.