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>

account/bp/account/routes.py

@@ -13,7 +13,7 @@ from sqlalchemy import select
 
 from shared.models import UserNewsletter
 from shared.infrastructure.fragments import fetch_fragments
-from shared.sx.helpers import sx_response, render_to_sx
+from shared.sx.helpers import sx_response, sx_call
 
 
 def register(url_prefix="/"):
@@ -66,7 +66,7 @@ def register(url_prefix="/"):
         translate = "translate-x-6" if un.subscribed else "translate-x-1"
         checked = "true" if un.subscribed else "false"
 
-        return sx_response(await render_to_sx(
+        return sx_response(sx_call(
             "account-newsletter-toggle",
             id=f"nl-{nid}", url=toggle_url,
             hdrs=f'{{"X-CSRFToken": "{csrf}"}}',