Make SxExpr a str subclass, sx_call/render functions return SxExpr

SxExpr is now a str subclass so it works everywhere a plain string
does (join, isinstance, f-strings) while serialize() still emits it
unquoted. sx_call() and all internal render functions (_render_to_sx,
async_eval_to_sx, etc.) return SxExpr, eliminating the "forgot to
wrap" bug class that caused the sx_content leak and list serialization
bugs.

- Phase 0: SxExpr(str) with .source property, __add__/__radd__
- Phase 1: sx_call returns SxExpr (drop-in, all 200+ sites unchanged)
- Phase 2: async_eval_to_sx, async_eval_slot_to_sx, _render_to_sx,
  mobile_menu_sx return SxExpr; remove isinstance(str) workaround
- Phase 3: Remove ~150 redundant SxExpr() wrappings across 45 files
- Phase 4: serialize() docstring, handler return docs, ;; returns: sx

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

orders/bp/checkout/routes.py

@@ -45,7 +45,7 @@ async def _render_checkout_return(ctx: dict, order=None, status: str = "",
                 else:
                     img = sx_call("order-item-no-image")
                 item_parts.append(sx_call("order-item-row",
-                    href=product_url, img=SxExpr(img),
+                    href=product_url, img=img,
                     title=item.product_title or "Unknown product",
                     pid=f"Product ID: {item.product_id}",
                     qty=f"Qty: {item.quantity}",
@@ -109,11 +109,11 @@ async def _render_checkout_return(ctx: dict, order=None, status: str = "",
             status_msg = sx_call("checkout-return-paid")
 
         content = sx_call("checkout-return-content",
-            summary=SxExpr(summary),
-            items=SxExpr(items) if items else None,
-            calendar=SxExpr(calendar) if calendar else None,
-            tickets=SxExpr(tickets) if tickets else None,
-            status_message=SxExpr(status_msg) if status_msg else None,
+            summary=summary,
+            items=items or None,
+            calendar=calendar or None,
+            tickets=tickets or None,
+            status_message=status_msg or None,
         )
 
     account_url = call_url(ctx, "account_url", "")

orders/bp/order/routes.py

@@ -71,7 +71,6 @@ def register() -> Blueprint:
         if not hosted_url:
             from shared.sx.page import get_template_context
             from shared.sx.helpers import sx_call, root_header_sx, header_child_sx, full_page_sx, call_url
-            from shared.sx.parser import SxExpr
             from shared.infrastructure.urls import cart_url
             tctx = await get_template_context()
             account_url = call_url(tctx, "account_url", "")
@@ -82,7 +81,7 @@ def register() -> Blueprint:
             content = sx_call(
                 "checkout-error-content",
                 msg="No hosted checkout URL returned from SumUp when trying to reopen payment.",
-                order=SxExpr(order_sx),
+                order=order_sx,
                 back_url=cart_url("/"),
             )
             html = await full_page_sx(tctx, header_rows=hdr, filter=filt, content=content)

orders/sx/handlers/account-nav-item.sx

@@ -1,4 +1,5 @@
 ;; Orders account-nav-item fragment handler
+;; returns: sx
 ;;
 ;; Renders the "orders" link for the account dashboard nav.