test: harden checked boundary coverage

Use DOM-aware rendered-text inspection for the v0 shell marker check and extend unkeyed h-for validation coverage from slots to handles.

req: test/005

req: scope/001

examples/v0/src/main.rs

@@ -248,7 +248,10 @@ mod tests {
         );
         assert_eq!(document.select(&selector("script[src=\"/slhx.js\"]")).count(), 1);
         assert_eq!(document.select(&selector("main[data-slhx-root=\"docs\"]")).count(), 1);
-        assert!(!html.contains("{+="));
+        assert!(
+            document.root_element().text().all(|text| !text.contains("{+=")),
+            "hemplate insertion markers must not leak into rendered text"
+        );
     }
 
     // req: html_safety/002 req: view/001 req: test/005