Expose shared core for iOS

[howardrock88]

What changed

• exposes TaskIslandCore and TaskIslandWidgetSupport as Swift package library products
• adds the iOS 17 platform declaration
• adds shared task recommendation, date-only due-date, focus-session, undo, and widget snapshot support

Why

The new standalone iOS repository must consume the platform-independent model and business logic from one source instead of copying it.

Validation

• clean Swift package build passed on macOS
• the standalone iOS project will pin this branch while the shared changes remain under review