Fire lifecycle event when widget modules are loaded

## Context

`packages/widget/src/configure-widget-modules.ts` has commented-out code and a `// TODO - fire event when widget is loaded` note, mirroring the `onAppModulesLoaded` event dispatch already implemented in `packages/app/src/configure-modules.ts`.

## Task

Implement an equivalent lifecycle event (e.g. `onWidgetModulesLoaded`) dispatched from `configureWidgetModules`, uncommenting/adapting the existing draft code, once the appropriate manifest/key field for widgets is confirmed.

_Filed while resolving `fusion-lint`'s `no-todo-without-issue` rule — see `packages/widget/src/configure-widget-modules.ts`._

Compare packages/widget/src/configure-widget-modules.ts with packages/app/src/configure-modules.ts, including the commented draft and the existing onAppModulesLoaded dispatch. Confirm the appropriate widget manifest/key field, then implement the equivalent event from configureWidgetModules. Done means the lifecycle event is dispatched after widget modules load and the no-todo-without-issue lint concern is resolved.

Written by the indexing model from the issue text.