Convert all 23 register_custom_layout calls to register_sx_layout across 6 services

Layout defcomps are now fully self-contained via IO-primitive auto-fetch
macros, eliminating Python layout functions that manually threaded context
values through SxExpr wrappers.

Services converted:
- Federation (1 layout): social
- Blog (7 layouts): blog, blog-settings, blog-cache, blog-snippets,
  blog-menu-items, blog-tag-groups, blog-tag-group-edit
- SX docs (2 layouts): sx, sx-section
- Cart (2 layouts): cart-page, cart-admin + orders/order-detail
- Events (9 layouts): calendar-admin, slots, slot, day-admin, entry,
  entry-admin, ticket-types, ticket-type, markets
- Market (2 layouts): market, market-admin

New IO primitives added to shared/sx/primitives_io.py:
- federation-actor-ctx, cart-page-ctx, request-view-args
- events-calendar-ctx, events-day-ctx, events-entry-ctx,
  events-slot-ctx, events-ticket-type-ctx
- market-header-ctx (pre-builds desktop/mobile nav as SxExpr)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cart/services/cart_page.py

@@ -172,6 +172,45 @@ class CartPageService:
             "summary": summary,
         }
 
+    async def admin_data(self, session, **kw):
+        """Populate post context for cart-admin layout headers."""
+        from quart import g
+        from shared.infrastructure.fragments import fetch_fragments
+
+        post = g.page_post
+        slug = post.slug if post else ""
+        post_id = post.id if post else None
+
+        # Fetch container_nav for post header
+        container_nav = ""
+        if post_id:
+            nav_params = {
+                "container_type": "page",
+                "container_id": str(post_id),
+                "post_slug": slug,
+            }
+            events_nav, market_nav = await fetch_fragments([
+                ("events", "container-nav", nav_params),
+                ("market", "container-nav", nav_params),
+            ], required=False)
+            container_nav = events_nav + market_nav
+
+        return {
+            "post": {
+                "id": post_id,
+                "slug": slug,
+                "title": (post.title if post else "")[:160],
+                "feature_image": getattr(post, "feature_image", None),
+            },
+            "container_nav": container_nav,
+        }
+
+    async def payments_admin_data(self, session, **kw):
+        """Admin data + payments data combined for cart-payments page."""
+        admin = await self.admin_data(session)
+        payments = await self.payments_data(session)
+        return {**admin, **payments}
+
     async def payments_data(self, session, **kw):
         from shared.sx.page import get_template_context