Add structuredClone() global function

Next.js hydration requires structuredClone(), which was not implemented.
Uses a JSON stringify/parse round-trip via V8 to deep-clone values,
covering all JSON-serializable types (objects, arrays, primitives).

src/browser/tests/window/window.html

@@ -115,6 +115,30 @@
   }
 </script>
 
+<script id=structuredClone>
+  // Basic types
+  testing.expectEqual(42, structuredClone(42));
+  testing.expectEqual('hello', structuredClone('hello'));
+  testing.expectEqual(true, structuredClone(true));
+  testing.expectEqual(null, structuredClone(null));
+
+  // Object deep clone
+  const obj = { a: 1, b: { c: 2 } };
+  const cloned = structuredClone(obj);
+  testing.expectEqual(1, cloned.a);
+  testing.expectEqual(2, cloned.b.c);
+  cloned.b.c = 99;
+  testing.expectEqual(2, obj.b.c); // original unchanged
+
+  // Array deep clone
+  const arr = [1, [2, 3]];
+  const clonedArr = structuredClone(arr);
+  testing.expectEqual(1, clonedArr[0]);
+  testing.expectEqual(2, clonedArr[1][0]);
+  clonedArr[1][0] = 99;
+  testing.expectEqual(2, arr[1][0]); // original unchanged
+</script>
+
 <script id=screen>
   testing.expectEqual(1920, screen.width);
   testing.expectEqual(1080, screen.height);

src/browser/webapi/Window.zig

@@ -412,6 +412,18 @@ pub fn atob(_: *const Window, input: []const u8, page: *Page) ![]const u8 {
     return decoded;
 }
 
+pub fn structuredClone(_: *const Window, value: js.Value) !js.Value {
+    // Simplified structured clone using JSON round-trip.
+    // Handles JSON-serializable types (objects, arrays, strings, numbers, booleans, null).
+    const local = value.local;
+    const str_handle = js.v8.v8__JSON__Stringify(local.handle, value.handle, null) orelse return error.DataCloneError;
+    const cloned_handle = js.v8.v8__JSON__Parse(local.handle, str_handle) orelse return error.DataCloneError;
+    return js.Value{
+        .local = local,
+        .handle = cloned_handle,
+    };
+}
+
 pub fn getFrame(self: *Window, idx: usize) !?*Window {
     const page = self._page;
     const frames = page.frames.items;
@@ -797,6 +809,7 @@ pub const JsApi = struct {
     pub const btoa = bridge.function(Window.btoa, .{});
     pub const atob = bridge.function(Window.atob, .{ .dom_exception = true });
     pub const reportError = bridge.function(Window.reportError, .{});
+    pub const structuredClone = bridge.function(Window.structuredClone, .{});
     pub const getComputedStyle = bridge.function(Window.getComputedStyle, .{});
     pub const getSelection = bridge.function(Window.getSelection, .{});