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>

market/app.py

@@ -228,6 +228,11 @@ def create_app() -> "Quart":
                 ("market", "container-nav", nav_params),
             ], required=False)
             ctx["container_nav"] = events_nav + market_nav
+            # Populate g._defpage_ctx for layout IO primitives
+            if not hasattr(g, '_defpage_ctx'):
+                g._defpage_ctx = {}
+            g._defpage_ctx.setdefault("post", post_data.get("post"))
+            g._defpage_ctx.setdefault("container_nav", ctx["container_nav"])
         return ctx
 
     # --- oEmbed endpoint ---