Migrate UI primitives from Radix to Base UI.

Co-authored-by: Cursor <[email protected]>

src/lib/as-child.ts

@@ -0,0 +1,22 @@
+import { isValidElement, type ReactNode } from "react";
+
+/**
+ * Map Radix-style `asChild` onto Base UI's `render` prop.
+ */
+export function resolveAsChild(
+	asChild: boolean | undefined,
+	children: ReactNode,
+	render?: any,
+): {
+	render?: any;
+	children: ReactNode;
+} {
+	if (render != null) return { render, children };
+	if (asChild && isValidElement(children)) {
+		return {
+			render: children,
+			children: (children.props as { children?: ReactNode }).children,
+		};
+	}
+	return { children };
+}