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>

shared/sx/primitives_io.py

@@ -40,6 +40,7 @@ IO_PRIMITIVES: frozenset[str] = frozenset({
     "request-path",
     "nav-tree",
     "get-children",
+    "g",
 })
 
 
@@ -300,6 +301,19 @@ async def _io_get_children(
 # Handler registry
 # ---------------------------------------------------------------------------
 
+async def _io_g(
+    args: list[Any], kwargs: dict[str, Any], ctx: RequestContext
+) -> Any:
+    """``(g "key")`` → getattr(g, key, None).
+
+    Reads a value from the Quart request-local ``g`` object.
+    Kebab-case keys are converted to snake_case automatically.
+    """
+    from quart import g
+    key = str(args[0]).replace("-", "_") if args else ""
+    return getattr(g, key, None)
+
+
 _IO_HANDLERS: dict[str, Any] = {
     "frag": _io_frag,
     "query": _io_query,
@@ -311,4 +325,5 @@ _IO_HANDLERS: dict[str, Any] = {
     "request-path": _io_request_path,
     "nav-tree": _io_nav_tree,
     "get-children": _io_get_children,
+    "g": _io_g,
 }