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>

federation/bp/social/routes.py

@@ -212,7 +212,6 @@ def register(url_prefix="/social"):
         """Re-render interaction buttons after a like/boost action."""
         from shared.models.federation import APInteraction
         from shared.browser.app.csrf import generate_csrf_token
-        from shared.sx.parser import SxExpr
         from sqlalchemy import select
 
         svc = services.federation
@@ -290,9 +289,9 @@ def register(url_prefix="/social"):
             count=str(boost_count))
 
         return sx_response(sx_call("federation-interaction-buttons",
-            like=SxExpr(like_form),
-            boost=SxExpr(boost_form),
-            reply=SxExpr(reply_sx) if reply_sx else None))
+            like=like_form,
+            boost=boost_form,
+            reply=reply_sx or None))
 
     # -- Following / Followers pagination --------------------------------------