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_entries/routes.py

@@ -259,7 +259,7 @@ def register():
         }
 
         from sxc.pages.renders import render_day_main_panel
-        html = await render_day_main_panel(ctx)
+        html = render_day_main_panel(ctx)
         mini_html = await fetch_fragment("cart", "cart-mini", params=frag_params, required=False)
         return sx_response(html + (mini_html or ""))
     
@@ -280,12 +280,12 @@ def register():
         day_slots = list(result.scalars())
 
         from sxc.pages.entries import render_entry_add_form
-        return sx_response(await render_entry_add_form(g.calendar, day, month, year, day_slots))
+        return sx_response(render_entry_add_form(g.calendar, day, month, year, day_slots))
 
     @bp.get("/add-button/")
     async def add_button(day: int, month: int, year: int, **kwargs):
         from sxc.pages.entries import render_entry_add_button
-        return sx_response(await render_entry_add_button(g.calendar, day, month, year))
+        return sx_response(render_entry_add_button(g.calendar, day, month, year))