App: single-instance + URL-scheme / deep-link handling

[codymullins]

Problem

A browser usually runs a single instance; clicking a link or launching with a URL should route to the running instance rather than spawn a second. Skyline has no single-instance or deep-link mechanism.

Current state

• No single-instance detection or deep-link event; each AppHost.Run() is independent.

What's needed

• A single-instance acquire API and a DeepLinkReceived(Uri) event; a launch with a URL forwards to the existing instance via IPC.
• macOS open-URL apple-event / NSDistributedNotificationCenter; Windows named pipe / WM_COPYDATA; Linux D-Bus or socket.

Acceptance criteria

• Second launch with a URL forwards to the first instance, which raises DeepLinkReceived and navigates.