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>

orders/sxc/pages/orders.sx

@@ -0,0 +1,27 @@
+;; Orders app — declarative page definitions
+
+;; ---------------------------------------------------------------------------
+;; Orders list
+;; ---------------------------------------------------------------------------
+
+(defpage orders-list
+  :path "/"
+  :auth :public
+  :layout (:orders
+    :list-url (orders-list-url))
+  :filter (orders-list-filter)
+  :aside (orders-list-aside)
+  :content (orders-list-content))
+
+;; ---------------------------------------------------------------------------
+;; Order detail
+;; ---------------------------------------------------------------------------
+
+(defpage order-detail
+  :path "/<int:order_id>/"
+  :auth :public
+  :layout (:order-detail
+    :list-url (order-list-url-from-detail)
+    :detail-url (order-detail-url))
+  :filter (order-detail-filter)
+  :content (order-detail-content))