feat(17-01): menu item type registry and dependent dropdown binding

- pact: HasMenuItemTypes, MenuItemType, MenuItemReference, MenuItemResolve,
  ResolvedMenuItem, MenuItemAlternate, DependentOptionsProvider,
  DependentOptionSet; Option gains Depth (omitempty)
- signpost (new module): boot-time registry with fail-loud key and pattern
  checks, layered URL patterns incl. PatternFrom for list types, host
  overrides at menu_items.patterns, Resolve and ReferenceOptions
- party: signpost.Activate after postcard, before Boot
- cabana: dependsOn on dropdowns (boot checks, controller-only), GET
  fields/{field}/dependent-options with swag, inventories, conformance case
- companions fixture, TestSignpostSmoke, TestCompanionsDependentSmoke, docs

modules/cabana/settings.go

@@ -89,6 +89,9 @@ func compileSettingWithTypes(pluginID string, item pact.SettingsItem, fsys fs.FS
 		if field.Preset != nil {
 			return nil, fmt.Errorf("cabana: setting %s field %s: preset is not supported on a settings form", item.Code, field.Name)
 		}
+		if err := controllerFormOnly(field); err != nil {
+			return nil, fmt.Errorf("cabana: setting %s field %s: %w", item.Code, field.Name, err)
+		}
 	}
 	form := &FormSchema{Name: item.Label, ModelClass: item.Model, Fields: fields}
 	columns := modelColumns(model)