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/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)