zk proposer: consider moving proof generation to a separate service

The resource requirements are very different when the proposer is generating a proof as compared to when it is just proposing (in optimistic mode). Consider splitting this into a separate service to be able to optimize.

Start by locating the Go entry points for the zk proposer and proof generation, then compare their resource requirements in optimistic mode and during proof generation. Determine whether a separate service boundary is appropriate and document the proposed split, including what work remains for a follow-up implementation.

Written by the indexing model from the issue text.