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>

blog/bp/menu_items/routes.py

@@ -144,7 +144,7 @@ def _render_page_search_results(pages, query, page, has_more) -> str:
     items_sx = "(<> " + " ".join(items) + ")"
     return sx_call("page-search-results",
                    items=SxExpr(items_sx),
-                   sentinel=SxExpr(sentinel) if sentinel else None)
+                   sentinel=sentinel or None)
 
 
 def _render_menu_items_nav_oob(menu_items) -> str:
@@ -191,12 +191,12 @@ def _render_menu_items_nav_oob(menu_items) -> str:
         if item_slug != "cart":
             item_parts.append(sx_call("blog-nav-item-link",
                 href=href, hx_get=f"/{item_slug}/", selected=selected,
-                nav_cls=nav_button_cls, img=SxExpr(img_sx), label=label,
+                nav_cls=nav_button_cls, img=img_sx, label=label,
             ))
         else:
             item_parts.append(sx_call("blog-nav-item-plain",
                 href=href, selected=selected, nav_cls=nav_button_cls,
-                img=SxExpr(img_sx), label=label,
+                img=img_sx, label=label,
             ))
 
     items_sx = "(<> " + " ".join(item_parts) + ")" if item_parts else ""