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/sxc/pages/helpers.py

@@ -279,11 +279,11 @@ async def _h_post_preview_content(slug=None, **kw):
     if preview.get("sx_rendered"):
         rendered_sx = sx_call("blog-preview-rendered", html=preview["sx_rendered"])
         sections.append(sx_call("blog-preview-section",
-            title="SX Rendered", content=SxExpr(rendered_sx)))
+            title="SX Rendered", content=rendered_sx))
     if preview.get("lex_rendered"):
         rendered_sx = sx_call("blog-preview-rendered", html=preview["lex_rendered"])
         sections.append(sx_call("blog-preview-section",
-            title="Lexical Rendered", content=SxExpr(rendered_sx)))
+            title="Lexical Rendered", content=rendered_sx))
 
     if not sections:
         return sx_call("blog-preview-empty")