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

examples/v0/src/main.rs

@@ -133,7 +133,7 @@ impl Hemplate for Todos {
     fn render_into(&self, buf: &mut String) -> Result<(), hemplate::error::HemplateError> {
         use std::fmt::Write as _;
 
-        buf.push_str("<section data-hemx-root=\"todos\">\n  <form data-hemx-handle=\"add_todo\" data-hemx-form=\"new_todo\">\n    <label>New todo\n      <input name=\"title\" required=\"required\">\n    </label>\n    <button type=\"submit\">Add</button>\n    <p data-hemx-error-for=\"title\"></p>\n  </form>\n\n  <p data-hemx-slot=\"summary\">");
+        buf.push_str("<section data-hemx-root=\"todos\">\n  <form data-hemx-handle=\"add_todo\" data-hemx-form=\"new_todo\">\n    <label>New todo\n      <input name=\"title\" required=\"required\">\n    </label>\n    <button type=\"submit\">Add</button>\n    <p data-hemx-error-for=\"title\"></p>\n  </form>\n\n  <p data-hemx-error role=\"status\" hidden=\"hidden\"></p>\n\n  <p data-hemx-slot=\"summary\">");
         write!(buf, "{}", hemplate::HtmlEscape(&self.summary))?;
         buf.push_str("</p>\n  <p data-hemx-slot=\"notice\">");
         write!(buf, "{}", hemplate::HtmlEscape(&self.notice))?;
@@ -627,6 +627,11 @@ mod tests {
             2,
             "add and rename forms expose generated field-error targets"
         );
+        assert_eq!(
+            document.select(&selector("[data-hemx-error]")).count(),
+            1,
+            "the app exposes one root-scoped transport error outlet"
+        );
     }
 
     // req: canonical_authoring/003 req: test/005

examples/v0/templates/todos.heml

@@ -4,6 +4,7 @@
     <button type="submit">Add</button>
     <p data-hemx-error-for="title"></p>
   </form>
+  <p data-hemx-error role="status" hidden="hidden"></p>
   <p data-hemx-slot="summary">{+ self.summary +}</p>
   <p data-hemx-slot="notice">{+ self.notice +}</p>
   <ul data-hemx-slot="todo_row">