Add the announcement of switched workspaces from Niri.

I haven't tested this with Braille support; I don't remember what I did with the cable.

internal/atspi/announcement_test.go

@@ -0,0 +1,18 @@
+// Copyright (c) 2026 Storm Dragon
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package atspi
+
+import "testing"
+
+func TestAnnouncementEventIsRegistered(t *testing.T) {
+	if AnnouncementEvent != "object:announcement" {
+		t.Fatalf("AnnouncementEvent = %q", AnnouncementEvent)
+	}
+	for _, eventType := range eventTypes {
+		if eventType == AnnouncementEvent {
+			return
+		}
+	}
+	t.Fatal("object:announcement is not in the registered event types")
+}