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/app.py

@@ -70,16 +70,22 @@ def create_app() -> "Quart":
         app.jinja_loader,
     ])
 
-    # Load orders-specific s-expression components
-    from sx.sx_components import load_orders_components
-    load_orders_components()
+    # Load orders-specific s-expression components (loaded at import time)
+    import sx.sx_components  # noqa: F811
+
+    # Setup defpage routes
+    from sxc.pages import setup_orders_pages
+    setup_orders_pages()
 
     app.register_blueprint(register_fragments())
     app.register_blueprint(register_actions())
     app.register_blueprint(register_data())
 
-    # Orders list at /
-    app.register_blueprint(register_orders(url_prefix="/"))
+    # Orders list at / (defpage routes mounted below)
+    bp = register_orders(url_prefix="/")
+    from shared.sx.pages import mount_pages
+    mount_pages(bp, "orders")
+    app.register_blueprint(bp)
 
     # Checkout webhook + return
     app.register_blueprint(register_checkout())