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/utils.py

@@ -110,7 +110,7 @@ def _product_detail_sx(d: dict, ctx: dict) -> str:
 
         gallery_inner = sx_call(
             "market-detail-gallery-inner",
-            like=SxExpr(like_sx) if like_sx else None,
+            like=like_sx or None,
             image=images[0], alt=d.get("title", ""),
             labels=SxExpr(labels_sx) if labels_sx else None,
             brand=brand,
@@ -123,8 +123,8 @@ def _product_detail_sx(d: dict, ctx: dict) -> str:
 
         gallery_sx = sx_call(
             "market-detail-gallery",
-            inner=SxExpr(gallery_inner),
-            nav=SxExpr(nav_buttons) if nav_buttons else None,
+            inner=gallery_inner,
+            nav=nav_buttons or None,
         )
 
         # Thumbnails
@@ -144,7 +144,7 @@ def _product_detail_sx(d: dict, ctx: dict) -> str:
         if user:
             like_sx = _like_button_sx(slug, liked_by_current_user, csrf, ctx)
         gallery_final = sx_call("market-detail-no-image",
-                                  like=SxExpr(like_sx) if like_sx else None)
+                                  like=like_sx or None)
 
     # Stickers below gallery
     stickers_sx = ""
@@ -206,8 +206,8 @@ def _product_detail_sx(d: dict, ctx: dict) -> str:
     return sx_call(
         "market-detail-layout",
         gallery=SxExpr(gallery_final),
-        stickers=SxExpr(stickers_sx) if stickers_sx else None,
-        details=SxExpr(details_sx),
+        stickers=stickers_sx or None,
+        details=details_sx,
     )