Migrate all apps to defpage declarative page routes

Replace Python GET page handlers with declarative defpage definitions in .sx
files across all 8 apps (sx docs, orders, account, market, cart, federation,
events, blog). Each app now has sxc/pages/ with setup functions, layout
registrations, page helpers, and .sx defpage declarations.

Core infrastructure: add g I/O primitive, PageDef support for auth/layout/
data/content/filter/aside/menu slots, post_author auth level, and custom
layout registration. Remove ~1400 lines of render_*_page/render_*_oob
boilerplate. Update all endpoint references in routes, sx_components, and
templates to defpage_* naming.

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

cart/sxc/pages/cart.sx

@@ -0,0 +1,25 @@
+;; Cart app defpage declarations.
+
+(defpage cart-overview
+  :path "/"
+  :auth :public
+  :layout :root
+  :content (overview-content))
+
+(defpage page-cart-view
+  :path "/"
+  :auth :public
+  :layout :cart-page
+  :content (page-cart-content))
+
+(defpage cart-admin
+  :path "/"
+  :auth :admin
+  :layout :cart-admin
+  :content (cart-admin-content))
+
+(defpage cart-payments
+  :path "/payments/"
+  :auth :admin
+  :layout (:cart-admin :selected "payments")
+  :content (cart-payments-content))