D3 first half: x_fc_workorder_count computes live count of active MRP
work orders linked to this SO (via mo.origin = so.name). Adds a
'Active WOs' smart button next to the existing PO / RFQ buttons on
the sale.order form. Clicking opens a filtered mrp.workorder list
grouped by MO.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>