How to protect high-bandwidth low-voltage lines from transients?
You just have to find the right part. For example, this NUP2114 TVS diode module has an advertised 0.8pF typical capacitance, and 1pF max. This module was designed for USB so it has 2 bi-directional data lines and 1 uni-directional line for the Vbus line, but you can find stand-alone bidirectional parts (or put 2 unidirectional parts in reverse bias) which meet your capacitance allowance.

Browsing quickly on Digikey brought up this ESD9B5.0ST5G single bi-directional TVS diode which has a typical 15pF capacitance.