Reliable transport handling in the terminal

This release makes atra's networking behaviour more predictable, both at your terminal and inside CI.

• Sharper protocol negotiation. atra now settles on the quickest available transport more consistently.
• Refined HTTP/3 support. Request handling over HTTP/3 has been improved so scripted POST and PUT calls behave the same across every protocol.
• Stronger connection security. Certificate validation now applies consistently across HTTP/1.1, HTTP/2 and HTTP/3.
• More helpful failures. When a connection can't be established, atra now points to the actual cause — a rejected certificate, a refused port — instead of a generic timeout.