https://ams.com/as5311#tab/description

Oskar from ODrive suggested the AS5311 linear magnetic encoder paired to a 128-pole magnetic ring.  In theory this is 262411 positions (19 bits), because it's a 12 bit sensor * 8 bits for each pole.  Unfortunately it's a relative encoder, so it's missing that critical absolute positioning I need. 

On the other hand, it's only ~$15 USD.  So if you're reading this and relative is enough for you... thank Oskar.