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>

federation/sxc/pages/social.sx

@@ -0,0 +1,49 @@
+;; Federation social pages
+
+(defpage home-timeline
+  :path "/"
+  :auth :login
+  :layout :social
+  :content (home-timeline-content))
+
+(defpage public-timeline
+  :path "/public"
+  :auth :public
+  :layout :social
+  :content (public-timeline-content))
+
+(defpage compose-form
+  :path "/compose"
+  :auth :login
+  :layout :social
+  :content (compose-content))
+
+(defpage search
+  :path "/search"
+  :auth :public
+  :layout :social
+  :content (search-content))
+
+(defpage following-list
+  :path "/following"
+  :auth :login
+  :layout :social
+  :content (following-content))
+
+(defpage followers-list
+  :path "/followers"
+  :auth :login
+  :layout :social
+  :content (followers-content))
+
+(defpage actor-timeline
+  :path "/actor/<int:id>"
+  :auth :public
+  :layout :social
+  :content (actor-timeline-content))
+
+(defpage notifications
+  :path "/notifications"
+  :auth :login
+  :layout :social
+  :content (notifications-content))