Russh: Pre-auth remote panic via all-zero Curve25519 peer public value (encode_mpint OOB)

A pre-authentication denial-of-service panic in russh 0.62.2 commit c4be19f1915c8682f4615c3fd50008512b474491, current default branch main as of 2026-07-22. An unauthenticated client sends a single SSHMSGKEXECDHINIT whose QC is 32 zero bytes. russh's Curve25519 KEX does not reject the all-zero pee...