Make shell execution explicit for literal mailto launches

Opt-in desktop port of resolved BCL Process.Start(string) calls with a well-formed absolute mailto literal containing a recipient, or explicit inline ProcessStartInfo(string) with that literal and no initializer. Share HTTP port binding/context safeguards; preserve the exact literal and caller evaluation while making UseShellExecute=true explicit. Leave variables, interpolations, concatenations, incomplete recipients, other schemes/overloads, existing settings, selector comments, unresolved/custom calls, lambda/query contexts, generic declarations and conditional snapshots unchanged. Not composed into upgrades; validate the target desktop's mail association.