feat(10.1-01): render header and form partials into an allowlisted node tree

- fields.yaml type: partial with a bare path name and config_list.yaml
  headerPartial resolve to {ConfigDir}/_{name}.htm, parsed at boot; the
  controller must implement pact.AdminPartialData
- html/template render against a curated view model, then x/net/html
  ParseFragment and a tag, attribute and URL allowlist with 64 KiB, 2000-node
  and depth-32 caps; the model type and trusted template types are refused
- GET .../partials/{name} with optional ?id= loaded through the form scope
- golang.org/x/net becomes a direct requirement (D-18), no new module

modules/cabana/settings.go

@@ -63,7 +63,7 @@ func compileSetting(pluginID string, item pact.SettingsItem, fsys fs.FS) (*Compi
 	}
 	for _, field := range fields {
 		// A settings screen has no admin controller to own actions or view models.
-		if field.Type == "widget" {
+		if field.Type == "widget" || field.Type == "partial" {
 			return nil, fmt.Errorf("cabana: setting %s field %s: type %s is not supported on a settings form", item.Code, field.Name, field.Type)
 		}
 	}