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>

addons/base/models/res_partner.go

@@ -20,7 +20,6 @@ func initResPartner() {
 	// -- Identity --
 	m.AddFields(
 		orm.Char("name", orm.FieldOpts{String: "Name", Required: true, Index: true}),
-		orm.Char("display_name", orm.FieldOpts{String: "Display Name", Compute: "_compute_display_name", Store: true}),
 		orm.Char("ref", orm.FieldOpts{String: "Reference", Index: true}),
 		orm.Selection("type", []orm.SelectionItem{
 			{Value: "contact", Label: "Contact"},