Add a SwiftUI-friendly presentation API

[HituziANDO]

Problem

Current built-in presentation APIs require UIKit or AppKit view-controller types and static calls. SwiftUI clients need presentation state that can be bound to a view hierarchy without searching for a global window.

Scope

• Provide a SwiftUI-facing adapter for structured update results.
• Expose presentation state suitable for alert, sheet, or a documented view modifier.
• Support iOS and macOS within their existing deployment targets where SwiftUI APIs permit.
• Do not introduce global window lookup.
• Add a minimal SwiftUI usage example while retaining the UIKit/AppKit APIs.

Dependencies

• Make built-in presentation scene-aware and MainActor-isolated #17
• Extract UpdateCheckService and add a structured async API #23

Acceptance criteria

• A SwiftUI application can check and present an update without creating a view controller.
• Presentation is scoped to the view or scene that owns the adapter.
• Optional and required update actions are represented correctly.
• The adapter remains testable without presenting system UI.

Tests

• Cover state transitions, dismissal, repeated results, scene ownership, and action routing.

Relevant files

• Framework/Sources/
• README.md
• iOSSample/
• MacSample/