Engineering Guide explaining the Cryptography of Leios

## Background
We need to document the cryptography of Leios, focusing on the voting and certification scheme used in Mithril.

## Scope
- Explain the goal of Mithril certificates (stake-based multisignatures).
- Describe the committee model (persistent vs non-persistent voters).
- Document the flow of operations (key registration, voting, certification, verification).
- Provide certificate size formula and examples.
- Contrast with the general Mithril protocol (Merkle paths vs on-chain registry).

## Deliverable
A structured engineering guide (Markdown document) to be included under a section titled **Cryptography of Leios**.