Wrap multi-expression sx returns in fragments to prevent kwarg truncation

When multiple sx expressions are concatenated and passed as a kwarg
value via SxExpr(), the parser only sees the first as the value — the
rest become extra args silently dropped by the component. Wrap in (<>)
fragments in render_editor_panel() and _page_cards_sx().

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

blog/sx/sx_components.py

@@ -364,7 +364,7 @@ def _page_cards_sx(ctx: dict) -> str:
     else:
         parts.append(sx_call("blog-no-pages"))
 
-    return "".join(parts)
+    return "(<> " + " ".join(parts) + ")" if parts else ""
 
 
 def _page_card_sx(page: dict, ctx: dict) -> str:
@@ -1267,7 +1267,7 @@ def render_editor_panel(save_error: str | None = None, is_page: bool = False) ->
     )
     parts.append(sx_call("blog-editor-scripts", js_src=editor_js, init_js=init_js))
 
-    return "".join(parts)
+    return "(<> " + " ".join(parts) + ")" if parts else ""
 
 
 # ---- New post/page ----