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>

account/sxc/pages/account.sx

@@ -0,0 +1,31 @@
+;; Account app — declarative page definitions
+
+;; ---------------------------------------------------------------------------
+;; Account dashboard
+;; ---------------------------------------------------------------------------
+
+(defpage account-dashboard
+  :path "/"
+  :auth :login
+  :layout :account
+  :content (account-content))
+
+;; ---------------------------------------------------------------------------
+;; Newsletters
+;; ---------------------------------------------------------------------------
+
+(defpage newsletters
+  :path "/newsletters/"
+  :auth :login
+  :layout :account
+  :content (newsletters-content))
+
+;; ---------------------------------------------------------------------------
+;; Fragment pages (tickets, bookings, etc. from events service)
+;; ---------------------------------------------------------------------------
+
+(defpage fragment-page
+  :path "/<slug>/"
+  :auth :login
+  :layout :account
+  :content (fragment-content))