Add _inherit (ExtendModel) + Inverse fields + sale extends partner

ORM:
- ExtendModel(name) retrieves existing model for extension (mirrors
  Python _inherit without _name). Panics on missing model.
- RegisterInverse(fieldName, fn) convenience for computed write-back
- Inverse field handling in Write(): caches new value, calls inverse
  method so computed fields can be written back

Sale module:
- Extends res.partner with sale_order_ids (O2M) and sale_order_count
  (computed) via ExtendModel — demonstrates real _inherit pattern

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

addons/sale/models/init.go

@@ -1,6 +1,41 @@
 package models
 
+import "odoo-go/pkg/orm"
+
 func Init() {
 	initSaleOrder()
 	initSaleOrderLine()
+	initResPartnerSaleExtension()
+}
+
+// initResPartnerSaleExtension extends res.partner with sale-specific fields.
+// Mirrors: odoo/addons/sale/models/res_partner.py
+//
+//	class ResPartner(models.Model):
+//	    _inherit = 'res.partner'
+//	    sale_order_count = fields.Integer(compute='_compute_sale_order_count')
+//	    sale_order_ids = fields.One2many('sale.order', 'partner_id', string='Sales Orders')
+func initResPartnerSaleExtension() {
+	partner := orm.ExtendModel("res.partner")
+	partner.AddFields(
+		orm.One2many("sale_order_ids", "sale.order", "partner_id", orm.FieldOpts{
+			String: "Sales Orders",
+		}),
+		orm.Integer("sale_order_count", orm.FieldOpts{
+			String:  "Sale Order Count",
+			Compute: "_compute_sale_order_count",
+		}),
+	)
+	partner.RegisterCompute("sale_order_count", func(rs *orm.Recordset) (orm.Values, error) {
+		env := rs.Env()
+		partnerID := rs.IDs()[0]
+		var count int
+		err := env.Tx().QueryRow(env.Ctx(),
+			`SELECT COUNT(*) FROM sale_order WHERE partner_id = $1`, partnerID,
+		).Scan(&count)
+		if err != nil {
+			count = 0
+		}
+		return orm.Values{"sale_order_count": count}, nil
+	})
 }