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/sxc/pages/cart.sx

@@ -32,12 +32,13 @@
   :path "/<page_slug>/admin/"
   :auth :admin
   :layout :cart-admin
+  :data (service "cart-page" "admin-data")
   :content (~cart-admin-content))
 
 (defpage cart-payments
   :path "/<page_slug>/admin/payments/"
   :auth :admin
   :layout (:cart-admin :selected "payments")
-  :data (service "cart-page" "payments-data")
+  :data (service "cart-page" "payments-admin-data")
   :content (~cart-payments-content
     :page-config page-config))