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>

sx/sxc/pages/docs.sx

@@ -15,6 +15,17 @@
 ;; Docs section
 ;; ---------------------------------------------------------------------------
 
+(defpage docs-index
+  :path "/docs/"
+  :auth :public
+  :layout (:sx-section
+    :section "Docs"
+    :sub-label "Docs"
+    :sub-href "/docs/introduction"
+    :sub-nav (docs-nav "Introduction")
+    :selected "Introduction")
+  :content (docs-content "introduction"))
+
 (defpage docs-page
   :path "/docs/<slug>"
   :auth :public
@@ -56,6 +67,17 @@
 ;; Protocols section
 ;; ---------------------------------------------------------------------------
 
+(defpage protocols-index
+  :path "/protocols/"
+  :auth :public
+  :layout (:sx-section
+    :section "Protocols"
+    :sub-label "Protocols"
+    :sub-href "/protocols/wire-format"
+    :sub-nav (protocols-nav "Wire Format")
+    :selected "Wire Format")
+  :content (protocol-content "wire-format"))
+
 (defpage protocol-page
   :path "/protocols/<slug>"
   :auth :public
@@ -71,6 +93,17 @@
 ;; Examples section
 ;; ---------------------------------------------------------------------------
 
+(defpage examples-index
+  :path "/examples/"
+  :auth :public
+  :layout (:sx-section
+    :section "Examples"
+    :sub-label "Examples"
+    :sub-href "/examples/click-to-load"
+    :sub-nav (examples-nav "Click to Load")
+    :selected "Click to Load")
+  :content (examples-content "click-to-load"))
+
 (defpage examples-page
   :path "/examples/<slug>"
   :auth :public
@@ -86,6 +119,17 @@
 ;; Essays section
 ;; ---------------------------------------------------------------------------
 
+(defpage essays-index
+  :path "/essays/"
+  :auth :public
+  :layout (:sx-section
+    :section "Essays"
+    :sub-label "Essays"
+    :sub-href "/essays/sx-sucks"
+    :sub-nav (essays-nav "sx sucks")
+    :selected "sx sucks")
+  :content (essay-content "sx-sucks"))
+
 (defpage essay-page
   :path "/essays/<slug>"
   :auth :public