v0.8.61 - Sort-by-column constraint

Generated tables now satisfy the Tabular engine's sort-by-column constraint: when column D has SortByColumn=S, every D value pairs with exactly one S value. Eliminates 'Cannot order [D] by [S] because at least one value in [D] has multiple distinct values in [S]' refresh warnings.

• ColumnConfig.sort_by_column populated from VPAX metadata
• Spark generator builds paired pool (round-robin) aligned to the sort column
• Topo-sort ensures the sort column is processed before its dependent
• README: new 'Sort-by columns' section