Combines header_row_span consecutive raw header rows into the single effective
header row build_header_keys/natural_column_keys expect, for spreadsheets whose
header spans more than one row (e.g. a merged “2015”/“2025” year row with a “Female”/
“Male” sub-label row underneath).
The natural (un-overridden) key for each column, exactly as build_header_keys would
derive it with no column overrides at all. Used as the matching target for column
overrides that reference a column by its source_key rather than by position.
Resolve a (possibly unordered, possibly sparse) list of column overrides against a
sheet’s natural header keys, producing one Column per natural column, aligned by index.