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>

cart/sxc/pages/renders.py

@@ -20,7 +20,7 @@ async def render_orders_page(ctx, orders, page, total_pages, search, search_coun
     header_rows = await render_to_sx_with_env("cart-orders-layout-full", {},
         list_url=list_url,
     )
-    filt = sx_call("order-list-header", search_mobile=SxExpr(await search_mobile_sx(ctx)))
+    filt = sx_call("order-list-header", search_mobile=await search_mobile_sx(ctx))
     return await full_page_sx(ctx, header_rows=header_rows, filter=filt,
                           aside=await search_desktop_sx(ctx), content=content)
 
@@ -44,7 +44,7 @@ def render_orders_rows(ctx, orders, page, total_pages, url_for_fn, qs_fn):
         next_scroll = sx_call("order-end-row")
     return sx_call("cart-orders-rows",
         rows=SxExpr("(<> " + " ".join(parts) + ")"),
-        next_scroll=SxExpr(next_scroll),
+        next_scroll=next_scroll,
     )
 
 
@@ -62,7 +62,7 @@ async def render_orders_oob(ctx, orders, page, total_pages, search, search_count
     oobs = await render_to_sx_with_env("cart-orders-layout-oob", {},
         list_url=list_url,
     )
-    filt = sx_call("order-list-header", search_mobile=SxExpr(await search_mobile_sx(ctx)))
+    filt = sx_call("order-list-header", search_mobile=await search_mobile_sx(ctx))
     return await oob_page_sx(oobs=oobs, filter=filt, aside=await search_desktop_sx(ctx), content=content)
 
 
@@ -116,7 +116,7 @@ async def render_checkout_error_page(ctx, error=None, order=None):
     hdr = await render_to_sx_with_env("layout-root-full", {})
     filt = sx_call("checkout-error-header")
     content = sx_call("checkout-error-content", msg=err_msg,
-        order=SxExpr(order_sx) if order_sx else None, back_url=cart_url("/"))
+        order=order_sx or None, back_url=cart_url("/"))
     return await full_page_sx(ctx, header_rows=hdr, filter=filt, content=content)