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>

federation/bp/auth/routes.py

@@ -44,11 +44,11 @@ ALLOWED_CLIENTS = {"blog", "market", "cart", "events", "account"}
 
 async def _render_social_auth_page(component: str, title: str, **kwargs) -> str:
     """Render an auth page with social layout — replaces sx_components helpers."""
-    from shared.sx.helpers import render_to_sx
+    from shared.sx.helpers import sx_call
     from shared.sx.page import get_template_context
     from sxc.pages.utils import _social_page
     ctx = await get_template_context()
-    content = await render_to_sx(component, **{k: v for k, v in kwargs.items() if v})
+    content = sx_call(component, **{k: v for k, v in kwargs.items() if v})
     return await _social_page(ctx, None, content=content, title=title)