Document recursion step of LaBRADOR

[omibo]

Summary

This PR adds docs/recursion.md, which walks through the recursion in Section 5.3 of the LaBRADOR paper. The document explains how the instance and witness for round k + 1 are produced from round k.

Note

Zero-padding is not yet considered.

How to review

1. Read Section 5.3 of the paper for context.
2. Open docs/recursion.md and verify that:

• every equation is addressed,
• the mathematical derivation is correct, and
• the terminology matches the paper.

Next Steps

• Document equation (1)
• Document equation (3)
• Document equation (4)
• Address zero-padding with concrete values