Stock variance measured from posted counts, attributed to each variant's default supplier

Reads system_quantity against counted_quantity on POSTED counts only, and puts each counted line to the variant's default supplier. Open counts are never read: an unfinished count is not a measurement.

Losses and gains are served separately and never netted, so there is no net field to reach for. A location that loses 500 and finds 480 has a counting problem, not a variance of 20. Nothing here is a shrink figure either, counting short can be theft, damage, a mis-shipment, a receiving error or a miscount, and counting long is found stock.

Every counted line lands in exactly one buckets entry, default_supplier, no_default_supplier or no_supplier_on_file, so the buckets sum to totals on every unit and line figure. The two unattributed buckets carry attribution_reason instead of a supplier and are never folded into one. On an attributed bucket, multi_supplier_line_count says how many of its lines came from variants carrying more than one supplier.

loss_cost and gain_cost are null, never 0, where there is nothing priceable, and cost_reason says why; *_units_unpriced says how many units the money leaves out even when it is populated. currency is null unless one denomination covers the lot.

A supplier with no posted count is not a supplier with clean stock: it is counted in measurement.suppliers_with_no_reading rather than served as a zero row. For the lines behind one count, use get_stocktake_lines_table.

Scope: stocktakes:read.