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/helpers.py

@@ -151,10 +151,9 @@ async def _h_page_markets_content(slug=None, **kw):
 async def _h_page_admin_content(slug=None, **kw):
     from shared.sx.page import get_template_context
     from shared.sx.helpers import sx_call
-    from shared.sx.parser import SxExpr
     ctx = await get_template_context()
     content = await _markets_admin_panel_sx(ctx)
-    return sx_call("market-admin-content-wrap", inner=SxExpr(content))
+    return sx_call("market-admin-content-wrap", inner=content)
 
 
 def _h_market_home_content(page_slug=None, market_slug=None, **kw):