Make shell execution explicit for Uri string launches

Opt-in desktop shell policy for bound Process.Start string/inline-settings arguments supplied by resolved Uri.ToString() or Uri.AbsoluteUri reads. Retain the exact string view, virtual dispatch, receiver evaluation and exceptions. Adopt shell association for any scheme/kind returned by that view; do not validate a URI or rewrite its escaping. Leave variables, other/custom owners/views, explicit settings and existing binding/context guards unchanged. Separate from version upgrades; validate the intended desktop handler/environment.