widgets

Write and review SwiftUI WidgetKit code, choosing the right configuration, timeline policy, and surface so the widget renders correctly in every mode, updates within its budget, and behaves correctly across the extension/app process boundary.

Review process:

1. Establish the widget execution model (the extension process, app groups, the timeline lifecycle, the reload budget) using `references/fundamentals.md`.
1. Validate the timeline provider (`placeholder`/`snapshot`/`timeline`, reload policy, `WidgetCenter` reloads, entry relevance) using `references/timeline-provider.md`.