fix(techdemo): rebootstrap inserted islands

Keep opaque island widgets live when slhx swaps or later DOM insertion adds a new island, and tear down the listener when an island leaves the document.

req: interop/001

req: examples/001

examples/techdemo/tests/e2e.rs

@@ -71,6 +71,8 @@ fn product_is_e2e_working_over_http() {
     assert!(island.header("content-type").contains("javascript"));
     assert!(island.text().contains("slhx:island-orbit"));
     assert!(island.text().contains("data-slhx-island"));
+    assert!(island.text().contains("MutationObserver"));
+    assert!(island.text().contains("removeEventListener"));
 
     let architecture = request("GET", "/architecture", &[("X-SLHX-Partial", "1")], "");
     assert_eq!(architecture.status, 200);