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/services/registry.py

@@ -15,6 +15,8 @@ Usage::
 """
 from __future__ import annotations
 
+from typing import Any
+
 from shared.contracts.protocols import (
     CalendarService,
     MarketService,
@@ -36,6 +38,7 @@ class _ServiceRegistry:
         self._market: MarketService | None = None
         self._cart: CartService | None = None
         self._federation: FederationService | None = None
+        self._extra: dict[str, Any] = {}
 
     # -- calendar -------------------------------------------------------------
     @property
@@ -81,10 +84,27 @@ class _ServiceRegistry:
     def federation(self, impl: FederationService) -> None:
         self._federation = impl
 
+    # -- generic registration --------------------------------------------------
+    def register(self, name: str, impl: Any) -> None:
+        """Register a service by name (for services without typed properties)."""
+        self._extra[name] = impl
+
+    def __getattr__(self, name: str) -> Any:
+        # Fallback to _extra dict for dynamically registered services
+        try:
+            extra = object.__getattribute__(self, "_extra")
+            if name in extra:
+                return extra[name]
+        except AttributeError:
+            pass
+        raise AttributeError(f"No service registered as: {name}")
+
     # -- introspection --------------------------------------------------------
     def has(self, name: str) -> bool:
         """Check whether a domain service is registered."""
-        return getattr(self, f"_{name}", None) is not None
+        if getattr(self, f"_{name}", None) is not None:
+            return True
+        return name in self._extra
 
 
 # Module-level singleton — import this everywhere.