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/layouts.py

@@ -3,8 +3,6 @@ from __future__ import annotations
 
 from typing import Any
 
-from shared.sx.parser import SxExpr
-
 
 def _register_cart_layouts() -> None:
     from shared.sx.layouts import register_custom_layout
@@ -80,8 +78,8 @@ def _page_cart_header_sx(ctx: dict, page_post: Any, *, oob: bool = False) -> str
         "menu-row-sx",
         id="page-cart-row", level=2, colour="sky",
         link_href=call_url(ctx, "cart_url", f"/{slug}/"),
-        link_label_content=SxExpr(label_sx),
-        nav=SxExpr(nav_sx), oob=oob,
+        link_label_content=label_sx,
+        nav=nav_sx, oob=oob,
     )
 
 
@@ -102,8 +100,8 @@ async def _cart_page_full(ctx: dict, **kw: Any) -> str:
     page_post = ctx.get("page_post")
     env = {}
     return await render_to_sx_with_env("cart-page-layout-full", env,
-        cart_row=SxExpr(_cart_header_sx(ctx)),
-        page_cart_row=SxExpr(_page_cart_header_sx(ctx, page_post)),
+        cart_row=_cart_header_sx(ctx),
+        page_cart_row=_page_cart_header_sx(ctx, page_post),
     )
 
 
@@ -112,9 +110,9 @@ async def _cart_page_oob(ctx: dict, **kw: Any) -> str:
     page_post = ctx.get("page_post")
     env = {}
     return await render_to_sx_with_env("cart-page-layout-oob", env,
-        root_header_oob=SxExpr(await root_header_sx(ctx, oob=True)),
-        cart_row_oob=SxExpr(_cart_header_sx(ctx, oob=True)),
-        page_cart_row=SxExpr(_page_cart_header_sx(ctx, page_post)),
+        root_header_oob=await root_header_sx(ctx, oob=True),
+        cart_row_oob=_cart_header_sx(ctx, oob=True),
+        page_cart_row=_page_cart_header_sx(ctx, page_post),
     )
 
 
@@ -124,8 +122,8 @@ async def _cart_admin_full(ctx: dict, **kw: Any) -> str:
     selected = kw.get("selected", "")
     env = {}
     return await render_to_sx_with_env("cart-admin-layout-full", env,
-        post_header=SxExpr(await _post_header_sx(ctx, page_post)),
-        admin_header=SxExpr(await _cart_page_admin_header_sx(ctx, page_post, selected=selected)),
+        post_header=await _post_header_sx(ctx, page_post),
+        admin_header=await _cart_page_admin_header_sx(ctx, page_post, selected=selected),
     )