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>

federation/sx/layouts.sx

@@ -1,17 +1,17 @@
-;; Federation layout defcomps — read ctx values from env free variables.
-;; `actor` is injected into env by the layout registration in __init__.py.
+;; Federation layout defcomps — fully self-contained via IO primitives.
+;; Registered via register_sx_layout("social", ...) in __init__.py.
 
 ;; Full page: root header + social header in header-child
 (defcomp ~social-layout-full ()
   (<> (~root-header-auto)
     (~header-child-sx
       :inner (~federation-social-header
-               :nav (~federation-social-nav :actor actor)))))
+               :nav (~federation-social-nav :actor (federation-actor-ctx))))))
 
 ;; OOB (HTMX): social header oob + root header oob
 (defcomp ~social-layout-oob ()
   (<> (~oob-header-sx
         :parent-id "root-header-child"
         :row (~federation-social-header
-               :nav (~federation-social-nav :actor actor)))
+               :nav (~federation-social-nav :actor (federation-actor-ctx))))
     (~root-header-auto true)))