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/calendar/admin/routes.py

@@ -19,7 +19,7 @@ def register():
     @require_admin
     async def calendar_description_edit(calendar_slug: str, **kwargs):
         from sxc.pages.renders import render_calendar_description_edit
-        html = await render_calendar_description_edit(g.calendar)
+        html = render_calendar_description_edit(g.calendar)
         return sx_response(html)
 
 
@@ -35,7 +35,7 @@ def register():
         await g.s.flush()
 
         from sxc.pages.renders import render_calendar_description
-        html = await render_calendar_description(g.calendar, oob=True)
+        html = render_calendar_description(g.calendar, oob=True)
         return sx_response(html)
 
 
@@ -43,7 +43,7 @@ def register():
     @require_admin
     async def calendar_description_view(calendar_slug: str, **kwargs):
         from sxc.pages.renders import render_calendar_description
-        html = await render_calendar_description(g.calendar)
+        html = render_calendar_description(g.calendar)
         return sx_response(html)
     
     return bp