Uncommitted sx-tools changes: WASM bundles, Playwright specs, engine fixes

WASM browser bundles rebuilt with latest kernel. Playwright test specs
updated (helpers, navigation, handler-responses, hypermedia-handlers,
isomorphic, SPA navigation). Engine/boot/orchestration SX files updated.
Handler examples and not-found page refreshed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/browser/sx-platform.js

@@ -413,6 +413,14 @@
             "hydrated:", !!islands[j]._sxBoundislandhydrated || !!islands[j]["_sxBound" + "island-hydrated"],
             "children:", islands[j].children.length);
         }
+        // Register popstate handler for back/forward navigation
+        window.addEventListener("popstate", function(e) {
+          var state = e.state;
+          var scrollY = (state && state.scrollY) ? state.scrollY : 0;
+          K.eval("(handle-popstate " + scrollY + ")");
+        });
+        // Signal boot complete
+        document.documentElement.setAttribute("data-sx-ready", "true");
         console.log("[sx] boot done");
       }
     }