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>

shared/sx/helpers.py

@@ -74,10 +74,10 @@ async def root_header_sx(ctx: dict, *, oob: bool = False) -> str:
     )
 
 
-def mobile_menu_sx(*sections: str) -> str:
+def mobile_menu_sx(*sections: str) -> SxExpr:
     """Assemble mobile menu from pre-built sections (deepest first)."""
     parts = [s for s in sections if s]
-    return "(<> " + " ".join(parts) + ")" if parts else ""
+    return SxExpr("(<> " + " ".join(parts) + ")") if parts else SxExpr("")
 
 
 async def mobile_root_nav_sx(ctx: dict) -> str:
@@ -96,13 +96,13 @@ async def mobile_root_nav_sx(ctx: dict) -> str:
 # Shared nav-item builders — used by BOTH desktop headers and mobile menus
 # ---------------------------------------------------------------------------
 
-async def _post_nav_items_sx(ctx: dict) -> str:
+async def _post_nav_items_sx(ctx: dict) -> SxExpr:
     """Build post-level nav items (container_nav + admin cog). Shared by
     ``post_header_sx`` (desktop) and ``post_mobile_nav_sx`` (mobile)."""
     post = ctx.get("post") or {}
     slug = post.get("slug", "")
     if not slug:
-        return ""
+        return SxExpr("")
     parts: list[str] = []
     page_cart_count = ctx.get("page_cart_count", 0)
     if page_cart_count and page_cart_count > 0:
@@ -130,11 +130,11 @@ async def _post_nav_items_sx(ctx: dict) -> str:
                                  is_admin_page=is_admin_page or None)
     if admin_nav:
         parts.append(admin_nav)
-    return "(<> " + " ".join(parts) + ")" if parts else ""
+    return SxExpr("(<> " + " ".join(parts) + ")") if parts else SxExpr("")
 
 
 async def _post_admin_nav_items_sx(ctx: dict, slug: str,
-                             selected: str = "") -> str:
+                             selected: str = "") -> SxExpr:
     """Build post-admin nav items (calendars, markets, etc.). Shared by
     ``post_admin_header_sx`` (desktop) and mobile menu."""
     select_colours = ctx.get("select_colours", "")
@@ -158,7 +158,7 @@ async def _post_admin_nav_items_sx(ctx: dict, slug: str,
         parts.append(await _render_to_sx("nav-link", href=href, label=label,
                              select_colours=select_colours,
                              is_selected=is_sel or None))
-    return "(<> " + " ".join(parts) + ")" if parts else ""
+    return SxExpr("(<> " + " ".join(parts) + ")") if parts else SxExpr("")
 
 
 # ---------------------------------------------------------------------------
@@ -177,7 +177,7 @@ async def post_mobile_nav_sx(ctx: dict) -> str:
         label=title,
         href=call_url(ctx, "blog_url", f"/{slug}/"),
         level=1,
-        items=SxExpr(nav),
+        items=nav,
     )
 
 
@@ -220,8 +220,8 @@ async def post_header_sx(ctx: dict, *, oob: bool = False, child: str = "") -> st
     return await _render_to_sx("menu-row-sx",
         id="post-row", level=1,
         link_href=link_href,
-        link_label_content=SxExpr(label_sx),
-        nav=SxExpr(nav_sx) if nav_sx else None,
+        link_label_content=label_sx,
+        nav=nav_sx,
         child_id="post-header-child",
         child=SxExpr(child) if child else None,
         oob=oob, external=True,
@@ -244,8 +244,8 @@ async def post_admin_header_sx(ctx: dict, slug: str, *, oob: bool = False,
     return await _render_to_sx("menu-row-sx",
         id="post-admin-row", level=2,
         link_href=admin_href,
-        link_label_content=SxExpr(label_sx),
-        nav=SxExpr(nav_sx) if nav_sx else None,
+        link_label_content=label_sx,
+        nav=nav_sx,
         child_id="post-admin-header-child", oob=oob,
     )
 
@@ -352,7 +352,7 @@ async def _render_to_sx_with_env(__name: str, extra_env: dict, **kwargs: Any) ->
     env = dict(get_component_env())
     env.update(extra_env)
     ctx = _get_request_context()
-    return await async_eval_slot_to_sx(ast, env, ctx)
+    return SxExpr(await async_eval_slot_to_sx(ast, env, ctx))
 
 
 async def _render_to_sx(__name: str, **kwargs: Any) -> str:
@@ -371,7 +371,7 @@ async def _render_to_sx(__name: str, **kwargs: Any) -> str:
     ast = _build_component_ast(__name, **kwargs)
     env = dict(get_component_env())
     ctx = _get_request_context()
-    return await async_eval_to_sx(ast, env, ctx)
+    return SxExpr(await async_eval_to_sx(ast, env, ctx))
 
 
 # Backwards-compat alias — layout infrastructure still imports this.
@@ -420,7 +420,7 @@ def sx_call(component_name: str, **kwargs: Any) -> str:
                 parts.append("(list " + " ".join(items) + ")")
         else:
             parts.append(serialize(val))
-    return "(" + " ".join(parts) + ")"
+    return SxExpr("(" + " ".join(parts) + ")")