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>

sx/sxc/pages/utils.py

@@ -68,7 +68,7 @@ def _sx_header_sx(nav: str | None = None, *, child: str | None = None) -> str:
     return sx_call("menu-row-sx",
         id="sx-row", level=1, colour="violet",
         link_href="/", link_label="sx",
-        link_label_content=SxExpr(label_sx),
+        link_label_content=label_sx,
         nav=SxExpr(nav) if nav else None,
         child_id="sx-header-child",
         child=SxExpr(child) if child else None,