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/types.py

@@ -213,9 +213,36 @@ class RelationDef:
     nav_label: str | None   # "markets"
 
 
+# ---------------------------------------------------------------------------
+# PageDef
+# ---------------------------------------------------------------------------
+
+@dataclass
+class PageDef:
+    """A declarative GET page defined in an .sx file.
+
+    Created by ``(defpage name :path "/..." :auth :public :content expr)``.
+    Slots are stored as unevaluated AST and resolved at request time.
+    """
+    name: str
+    path: str
+    auth: str | list            # "public", "login", "admin", or ["rights", ...]
+    layout: Any                 # layout name/config (unevaluated)
+    cache: dict | None
+    data_expr: Any              # unevaluated AST
+    content_expr: Any           # unevaluated AST
+    filter_expr: Any
+    aside_expr: Any
+    menu_expr: Any
+    closure: dict[str, Any] = field(default_factory=dict)
+
+    def __repr__(self):
+        return f"<page:{self.name} path={self.path!r}>"
+
+
 # ---------------------------------------------------------------------------
 # Type alias
 # ---------------------------------------------------------------------------
 
 # An s-expression value after evaluation
-SExp = int | float | str | bool | Symbol | Keyword | Lambda | Macro | Component | HandlerDef | RelationDef | list | dict | _Nil | None
+SExp = int | float | str | bool | Symbol | Keyword | Lambda | Macro | Component | HandlerDef | RelationDef | PageDef | list | dict | _Nil | None