Desktop deep links, end to end

A desktop deep link has to survive four independent hops, and each one can drop it silently:

1. Registration — the OS must know your scheme belongs to your executable.
2. Launch/delivery — the OS hands the URL to the process, differently on every platform.
3. The app's own filter — startup code decides which argument is a link.
4. App state — the URL has to reach the piece of state that was waiting for it.

A dropped link almost never produces an error. The app comes to the foreground and looks fine, which is why these bugs get filed as "the redirect doesn't work" and looked for in the browser.

Registration, per platform

Three different mechanisms; every scheme you support must be present in all three, and adding a second scheme later is exactly when one gets forgotten.