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/slots/routes.py

@@ -111,19 +111,19 @@ def register():
         # Success → re-render the slots table
         slots = await svc_list_slots(g.s, g.calendar.id)
         from sxc.pages.slots import render_slots_table
-        return sx_response(await render_slots_table(slots, g.calendar))
+        return sx_response(render_slots_table(slots, g.calendar))
 
         
     @bp.get("/add")
     @require_admin
     async def add_form(**kwargs):
         from sxc.pages.slots import render_slot_add_form
-        return sx_response(await render_slot_add_form(g.calendar))
+        return sx_response(render_slot_add_form(g.calendar))
 
     @bp.get("/add-button")
     @require_admin
     async def add_button(**kwargs):
         from sxc.pages.slots import render_slot_add_button
-        return sx_response(await render_slot_add_button(g.calendar))
+        return sx_response(render_slot_add_button(g.calendar))
 
     return bp