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>

events/bp/calendars/routes.py

@@ -69,7 +69,7 @@ def register():
         from shared.sx.page import get_template_context
         from sxc.pages.renders import render_calendars_list_panel
         ctx = await get_template_context()
-        html = await render_calendars_list_panel(ctx)
+        html = render_calendars_list_panel(ctx)
 
         # Blog-embedded mode: also update post nav
         if post_data:
@@ -85,7 +85,7 @@ def register():
             ).scalars().all()
 
             associated_entries = await get_associated_entries(post_id)
-            nav_oob = await render_post_nav_entries_oob(associated_entries, cals, post_data["post"])
+            nav_oob = render_post_nav_entries_oob(associated_entries, cals, post_data["post"])
             html = html + nav_oob
 
         return sx_response(html)