fix(runtime): show transport failures in app error outlets

Make failed hemx requests visibly recoverable by updating root-scoped data-hemx-error outlets while preserving fail-closed non-2xx handling and inspectable hemx:error events. The v0 generated-app exemplar now includes a transport error outlet alongside generated field-error targets.

req: runtime/005

req: examples/001

req: canonical_authoring/003

hemx-build/src/lib.rs

@@ -1541,6 +1541,7 @@ fn known_hemx_attr(name: &str) -> bool {
             | "data-hemx-nav"
             | "data-hemx-boost"
             | "data-hemx-error-for"
+            | "data-hemx-error"
             | "data-hemx-island"
     )
 }