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>

events/sxc/pages/utils.py

@@ -2,7 +2,6 @@
 from __future__ import annotations
 
 from shared.sx.helpers import sx_call
-from shared.sx.parser import SxExpr
 
 
 # ---------------------------------------------------------------------------
@@ -146,7 +145,7 @@ def _view_toggle_html(ctx: dict, view: str) -> str:
                   list_href=list_href, tile_href=tile_href,
                   hx_select=hx_select, list_cls=list_active,
                   tile_cls=tile_active, storage_key="events_view",
-                  list_svg=SxExpr(_get_list_svg()), tile_svg=SxExpr(_get_tile_svg()))
+                  list_svg=_get_list_svg(), tile_svg=_get_tile_svg())
 
 
 def _cart_icon_oob(count: int) -> str: