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/page_markets/routes.py

@@ -32,7 +32,7 @@ def register() -> Blueprint:
 
         from sxc.pages.renders import render_page_markets_cards
         post_slug = post.get("slug", "")
-        sx_src = await render_page_markets_cards(markets, has_more, page, post_slug)
+        sx_src = render_page_markets_cards(markets, has_more, page, post_slug)
         return sx_response(sx_src)
 
     return bp