Valuation layers for unbuild orders

This module extends the unbuild order screen in the Manufacturing app. On the form of an unbuild order (mrp.unbuild) a new "Valuation" smart button appears in the button box at the top right, with a dollar icon.

• The button is only shown once the unbuild order is no longer in draft state.
• It is restricted to users with the Inventory manager right (or the technical no-one group).
• Clicking it opens the standard Stock Valuation Layers list, filtered to only the valuation entries linked to the moves of that unbuild order (both the consumed components and the produced product).
• The list opens without the usual date filter and without grouping by product, so all related layers are visible at once.

No new fields, no automatic calculations, no emails or scheduled jobs are added. The module only adds a shortcut from the unbuild order to the existing valuation data managed by the Inventory accounting features (stock_account), which is why it depends on mrp_account.