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>

pkg/orm/model.go

@@ -276,6 +276,39 @@ func (m *Model) Extend(fields ...*Field) *Model {
 	return m
 }
 
+// ExtendModel retrieves an existing model for extension by another module.
+// Mirrors: Python Odoo's _inherit = 'model.name' (without _name).
+//
+// This is the formal mechanism for cross-module model extension. It provides:
+//  1. Explicit intent (extending vs creating)
+//  2. Panic on missing model (catch registration order bugs early)
+//  3. Python Odoo compatibility pattern
+//
+// Usage:
+//
+//	m := orm.ExtendModel("res.partner")
+//	m.AddFields(orm.Char("industry", orm.FieldOpts{String: "Industry"}))
+//	m.RegisterMethod("action_custom", func(rs *orm.Recordset, args ...interface{}) (interface{}, error) { ... })
+func ExtendModel(name string) *Model {
+	m := Registry.Get(name)
+	if m == nil {
+		panic(fmt.Sprintf("orm: cannot extend unregistered model %q", name))
+	}
+	return m
+}
+
+// RegisterInverse registers an inverse method for a computed field.
+// The method is auto-named "_inverse_<fieldName>" and linked to the field's
+// Inverse property so that Write() calls it automatically.
+// Mirrors: odoo/orm/fields.py Field.inverse
+func (m *Model) RegisterInverse(fieldName string, fn MethodFunc) {
+	inverseName := "_inverse_" + fieldName
+	m.RegisterMethod(inverseName, fn)
+	if f := m.GetField(fieldName); f != nil {
+		f.Inverse = inverseName
+	}
+}
+
 // CreateTableSQL generates the CREATE TABLE statement for this model.
 // Mirrors: odoo/orm/models.py BaseModel._table_exist / init()
 func (m *Model) CreateTableSQL() string {