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>

orders/bp/orders/routes.py

@@ -73,7 +73,7 @@ def register(url_prefix: str) -> Blueprint:
         result = await g.s.execute(stmt)
         orders = result.scalars().all()
 
-        from shared.sx.helpers import sx_response, render_to_sx
+        from shared.sx.helpers import sx_response, sx_call
         from shared.utils import route_prefix
 
         pfx = route_prefix()
@@ -95,16 +95,16 @@ def register(url_prefix: str) -> Blueprint:
         # Build just the rows fragment (not full table) for infinite scroll
         parts = []
         for od in order_dicts:
-            parts.append(await render_to_sx("order-row-pair",
+            parts.append(sx_call("order-row-pair",
                 order=od,
                 detail_url_prefix=detail_prefix))
         if page < total_pages:
-            parts.append(await render_to_sx("infinite-scroll",
+            parts.append(sx_call("infinite-scroll",
                 url=rows_url + qs_fn(page=page + 1),
                 page=page, total_pages=total_pages,
                 id_prefix="orders", colspan=5))
         else:
-            parts.append(await render_to_sx("order-end-row"))
+            parts.append(sx_call("order-end-row"))
         sx_src = "(<> " + " ".join(parts) + ")"
 
         resp = sx_response(sx_src)