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/post/admin/routes.py

@@ -226,7 +226,7 @@ def _render_associated_entries(all_calendars, associated_entry_ids, post_slug: s
                 confirm_text=f"This will remove {e_name} from this post",
                 toggle_url=toggle_url,
                 hx_headers=f'{{"X-CSRFToken": "{csrf}"}}',
-                img=SxExpr(img_sx), name=e_name,
+                img=img_sx, name=e_name,
                 date_str=f"{cal_name} \u2022 {date_str}",
             ))
 
@@ -237,7 +237,7 @@ def _render_associated_entries(all_calendars, associated_entry_ids, post_slug: s
     else:
         content_sx = sx_call("blog-associated-entries-empty")
 
-    return sx_call("blog-associated-entries-panel", content=SxExpr(content_sx))
+    return sx_call("blog-associated-entries-panel", content=content_sx)
 
 
 def _render_nav_entries_oob(associated_entries, calendars, post: dict) -> str: