Fix P0-P2 bugs: sale.order M2M, display_name, DefaultGet

P0: Fix sale.order creation (was completely broken)
- Corrected M2M junction table name from sale_order_line_account_tax_rel
  to account_tax_sale_order_line_rel (ORM sorts alphabetically)
- Added fallback in BeforeCreate if sequence generation fails

P1: Add display_name as magic field on ALL models
- Added to addMagicFields() in pkg/orm/model.go (like Python BaseModel)
- Computed on-the-fly in Read() from recName field, no DB column
- Removed explicit display_name from res.partner (now auto-inherited)

P2: Add DefaultGet hooks for sale.order and purchase.order
- Sets company_id, currency_id, date_order/date_planned from environment
- Follows same pattern as account.move's DefaultGet

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

pkg/orm/model.go

@@ -129,6 +129,13 @@ func (m *Model) addMagicFields() {
 		Readonly: true,
 	}))
 
+	// display_name: computed on-the-fly from rec_name, not stored in DB.
+	// Mirrors: odoo/orm/models.py BaseModel.display_name (computed field on ALL models)
+	m.AddField(Char("display_name", FieldOpts{
+		String:  "Display Name",
+		Compute: "_compute_display_name",
+	}))
+
 	if m.logAccess {
 		m.AddField(Many2one("create_uid", "res.users", FieldOpts{
 			String:   "Created by",