SMTP and IMAP clients now expose a BindIP *net.TCPAddr field. When set,
outbound TCP binds to that IP; otherwise the netbind helper falls back to
WORKER_BIND_IP env var, then default route.
IMAP switched from raw tls.Dial to netbind.TLSDialer so the source IP is
configurable; behavior identical when BindIP is nil.
No call-site changes required: backwards compatible.