feat(configurator): Phase D batch 2 — active WOs stat button on SO form

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>

fusion_plating/fusion_plating_configurator/views/sale_order_views.xml

@@ -33,6 +33,14 @@
                         <span class="o_stat_text">PO</span>
                     </div>
                 </button>
+                <button name="action_view_workorders"
+                        type="object"
+                        class="oe_stat_button"
+                        icon="fa-cogs"
+                        invisible="x_fc_workorder_count == 0">
+                    <field name="x_fc_workorder_count" widget="statinfo"
+                           string="Active WOs"/>
+                </button>
             </xpath>
             <xpath expr="//notebook" position="inside">
                 <page string="Plating" name="plating_tab">