fix(js): expose runtime API before startup

Assign window.hemx before startup side effects so constrained browser contexts can still inspect the runtime and root state even if bootstrap work reports errors.

req: runtime/002

hemx-js/tests/runtime.rs

@@ -1,3 +1,19 @@
+#[test]
+fn runtime_exposes_debug_api_before_startup_side_effects() {
+    // req: runtime/002
+    let source = hemx_js::RUNTIME_JS;
+
+    let api = source
+        .find("window.hemx = Object.freeze")
+        .expect("runtime exposes browser API");
+    let start = source
+        .find("if (document.readyState === \"loading\") document.addEventListener(\"DOMContentLoaded\", start)")
+        .expect("runtime starts after declaration");
+    assert!(api < start);
+    assert!(source.contains("try {\n        bootstrapState(root);"));
+    assert!(source.contains("emit(root, \"hemx:error\", String(error));"));
+}
+
 #[test]
 fn runtime_posts_urlencoded_forms_by_default() {
     let source = hemx_js::RUNTIME_JS;