DesktopPermissionHandler

Common interface for JVM desktop permission handlers (macOS TCC today; a future Windows-equivalent, if one is ever added, would implement this too).

Mirrors PermissionHandler (iOS) in shape, not by sharing the type: the iOS interface documents a main-thread requirement that has no JVM equivalent, and iOS/desktop handlers are registered into separate registries with separate opt-in modules, so keeping them distinct types avoids conflating two unrelated platforms' constraints.