proposer: Support restarts without losing in-flight proofs

Currently `kona-sp1-proposer` tracks in-flight proof requests in memory, but if it's restarted it loses track of in-flight proofs which wastes time and money.

This issue tracks future work to make this durable across proposer restarts. (Check if disk is good enough or if it's ephemeral).

Start by examining how kona-sp1-proposer tracks in-flight proof requests and what happens during a proposer restart. Determine whether disk persistence is sufficient or the state is ephemeral; done means in-flight proofs remain tracked across restarts without wasted work or cost.

Written by the indexing model from the issue text.