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>

shared/sx/pages.py

@@ -184,7 +184,9 @@ async def execute_page(
     if page_def.data_expr is not None:
         data_result = await async_eval(page_def.data_expr, env, ctx)
         if isinstance(data_result, dict):
-            env.update(data_result)
+            # Merge with kebab-case keys so SX symbols can reference them
+            for k, v in data_result.items():
+                env[k.replace("_", "-")] = v
 
     # Render content slot (required)
     content_sx = await _eval_slot(page_def.content_expr, env, ctx)