Standardize action and region loading states

Keep progress inside the initiating Button/IconButton or centered within a
LoadingRegion without changing layout or unmounting live content. Document
ownership and completion/error behavior in the registry and living catalog.

Validation: qualified NET02 DG build/typecheck/registry/loading tests;
existing browser geometry, theme and error lifecycle acceptance. All eleven
committed files match the qualified source artifact byte for byte.

packages/ui-react/src/index.ts

@@ -1,5 +1,6 @@
 export * from "./AppHeader.js";
 export * from "./ActivityIndicator.js";
+export * from "./LoadingRegion.js";
 export * from "./AdminNavigationPanel.js";
 export * from "./ApplicationShell.js";
 export * from "./ApplicationSidePanel.js";