Sale margin sync

This module links stock valuation entries to sales order lines and refreshes the cost used to calculate margin.

• When a new stock valuation entry is created for a stock move that comes from a sales order line, the module immediately recalculates the cost on that sales order line.
• When the unit cost of an existing stock valuation entry is modified, the same recalculation is triggered.
• Only outgoing stock movements (negative quantities) are synchronised, so customer deliveries drive the cost update.
• If the unit of measure of the valuation entry differs from the unit of measure on the sales order line, the cost is converted before being written.
• The recalculated value is stored as the purchase price on the sales order line, which is the field used by the margin calculation.
• A technical flag allows other processes to skip this synchronisation when needed.

It touches the stock.valuation.layer model and works together with the sale margin and sale stock features. There are no new menus or screens: the change happens silently in the background on sales order lines.