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/layouts.py

@@ -16,30 +16,27 @@ def _register_sx_layouts() -> None:
 async def _sx_full_headers(ctx: dict, **kw: Any) -> str:
     """Full headers for sx home page: root + sx menu row."""
     from shared.sx.helpers import render_to_sx_with_env
-    from shared.sx.parser import SxExpr
 
     main_nav = _main_nav_sx(kw.get("section"))
     sx_row = _sx_header_sx(main_nav)
     return await render_to_sx_with_env("sx-layout-full", {},
-        sx_row=SxExpr(sx_row))
+        sx_row=sx_row)
 
 
 async def _sx_oob_headers(ctx: dict, **kw: Any) -> str:
     """OOB headers for sx home page."""
     from shared.sx.helpers import render_to_sx_with_env, oob_header_sx
-    from shared.sx.parser import SxExpr
 
     main_nav = _main_nav_sx(kw.get("section"))
     sx_row = _sx_header_sx(main_nav)
     rows = await render_to_sx_with_env("sx-layout-full", {},
-        sx_row=SxExpr(sx_row))
+        sx_row=sx_row)
     return await oob_header_sx("root-header-child", "sx-header-child", rows)
 
 
 async def _sx_section_full_headers(ctx: dict, **kw: Any) -> str:
     """Full headers for sx section pages: root + sx row + sub row."""
     from shared.sx.helpers import render_to_sx_with_env
-    from shared.sx.parser import SxExpr
 
     section = kw.get("section", "")
     sub_label = kw.get("sub_label", section)
@@ -51,13 +48,12 @@ async def _sx_section_full_headers(ctx: dict, **kw: Any) -> str:
     sub_row = _sub_row_sx(sub_label, sub_href, sub_nav, selected)
     sx_row = _sx_header_sx(main_nav, child=sub_row)
     return await render_to_sx_with_env("sx-section-layout-full", {},
-        sx_row=SxExpr(sx_row))
+        sx_row=sx_row)
 
 
 async def _sx_section_oob_headers(ctx: dict, **kw: Any) -> str:
     """OOB headers for sx section pages."""
     from shared.sx.helpers import render_to_sx_with_env, oob_header_sx
-    from shared.sx.parser import SxExpr
 
     section = kw.get("section", "")
     sub_label = kw.get("sub_label", section)
@@ -69,7 +65,7 @@ async def _sx_section_oob_headers(ctx: dict, **kw: Any) -> str:
     sub_row = _sub_row_sx(sub_label, sub_href, sub_nav, selected)
     sx_row = _sx_header_sx(main_nav, child=sub_row)
     rows = await render_to_sx_with_env("sx-section-layout-full", {},
-        sx_row=SxExpr(sx_row))
+        sx_row=sx_row)
     return await oob_header_sx("root-header-child", "sx-header-child", rows)
 
 

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,