Phase 0 exploration for alknet-ssh: confirms SSH-over-QUIC-bistream via
tokio::io::join (no custom adapter needed, per reference impl), russh 0.60.2
generic run_stream/connect_stream, and channel-into-bistream multiplexing.
Surfaces 9 decision points for Phase 1: host key sourcing (vault-derived vs
config), channel policy v1 surface, client + SOCKS5 crate split, crypto
backend, auth method coverage, and a stream-handling POC to close russh's
upstream test gap.