Eliminate Python page helpers from orders — pure .sx defpages with IO primitives

Orders defpages now fetch data via (service ...) and generate URLs via
(url-for ...) and (route-prefix) directly in .sx. No Python middleman.

- Add url-for, route-prefix IO primitives to shared/sx/primitives_io.py
- Add generic register()/\_\_getattr\_\_ to ServiceRegistry for dynamic services
- Create OrdersPageService with list_page_data/detail_page_data methods
- Rewrite orders.sx defpages to use IO primitives + defcomp calls
- Remove ~320 lines of Python page helpers from orders/sxc/pages/__init__.py
- Convert :data env merge to use kebab-case keys for SX symbol access

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

orders/services/__init__.py

@@ -4,3 +4,6 @@ from __future__ import annotations
 
 def register_domain_services() -> None:
     """Register services for the orders app."""
+    from shared.services.registry import services
+    from .orders_page import OrdersPageService
+    services.register("orders_page", OrdersPageService())