Documentation for auction: user guide and operational bounds

## Context
Separated from #253 to track documentation work independently from the audit.

## Done Criteria
1. There is user documentation in the readme (e.g., how do parties participate?)
2. There is operational documentation helping developers understand the bounds of the system (e.g., what is the worst that can happen, how do we ensure there are auction participants, what happens if none shows up).

## Why Important
Users and operators need clear guidance on how the auction works, how to participate, and what edge cases to expect

## Notes
- The audit work is tracked separately in #253.

Start with the README and the auction implementation, tracing how parties participate and what happens when no participant appears. Document user participation, operational bounds, worst-case outcomes, and no-participant behavior; done means the README covers both the user guide and developer-facing edge cases described in the issue.

Written by the indexing model from the issue text.