Extract the dial-then-handshake sequence into nbssh.Handshake, which
applies the context deadline to the socket for the duration of the
handshake. Previously only the Android client did this; the CLI, wasm
and SSH proxy paths could block forever on a peer that accepts the TCP
connection and then goes silent, since ClientConfig.Timeout is not used
by NewClientConn.
Extract the identical PTY session setup shared by the wasm and Android
terminal clients into ssh.StartPTYSession, and move the stored-key host
verification onto the engine so the embed client delegates and the
Android client passes the engine directly as HostKeyVerifier.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
