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>

market/sxc/pages/cards.py

@@ -188,9 +188,9 @@ def _market_card_sx(market: Any, page_info: dict, *, show_page_badge: bool = Tru
 
     return sx_call(
         "market-market-card",
-        title_content=SxExpr(title_sx) if title_sx else None,
-        desc_content=SxExpr(desc_sx) if desc_sx else None,
-        badge_content=SxExpr(badge_sx) if badge_sx else None,
+        title_content=title_sx or None,
+        desc_content=desc_sx or None,
+        badge_content=badge_sx or None,
     )