Remove render_to_sx from public API: enforce sx_call for all service code

Replace ~250 render_to_sx calls across all services with sync sx_call,
converting many async functions to sync where no other awaits remained.
Make render_to_sx/render_to_sx_with_env private (_render_to_sx).
Add (post-header-ctx) IO primitive and shared post/post-admin defmacros.
Convert built-in post/post-admin layouts from Python to register_sx_layout
with .sx defcomps. Remove dead post_admin_mobile_nav_sx.

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

cart/bp/orders/routes.py

@@ -154,7 +154,7 @@ def register(url_prefix: str) -> Blueprint:
             )
             resp = await make_response(html)
         elif page > 1:
-            sx_src = await render_orders_rows(
+            sx_src = render_orders_rows(
                 ctx, orders, page, total_pages, url_for, qs_fn,
             )
             resp = sx_response(sx_src)

cart/bp/page_admin/routes.py

@@ -49,7 +49,7 @@ def register():
         from shared.sx.page import get_template_context
         from sxc.pages.renders import render_cart_payments_panel
         ctx = await get_template_context()
-        html = await render_cart_payments_panel(ctx)
+        html = render_cart_payments_panel(ctx)
         return sx_response(html)
 
     return bp