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>

market/bp/product/routes.py

@@ -129,7 +129,7 @@ def register():
       from sxc.pages.renders import render_like_toggle_button
 
       if not g.user:
-          return sx_response(await render_like_toggle_button(product_slug, False), status=403)
+          return sx_response(render_like_toggle_button(product_slug, False), status=403)
 
       user_id = g.user.id
 
@@ -138,7 +138,7 @@ def register():
       })
       liked = result["liked"]
 
-      return sx_response(await render_like_toggle_button(product_slug, liked))
+      return sx_response(render_like_toggle_button(product_slug, liked))
 
 
 
@@ -257,7 +257,7 @@ def register():
             from sxc.pages.renders import render_cart_added_response
             item_data = getattr(g, "item_data", {})
             d = item_data.get("d", {})
-            return sx_response(await render_cart_added_response(g.cart, ci_ns, d))
+            return sx_response(render_cart_added_response(g.cart, ci_ns, d))
 
         # normal POST: go to cart page
         from shared.infrastructure.urls import cart_url