A multi-attribute negotiation extreme strategy

Description

The pitch.

## Summary

This PR adds a multi-attribute negotiation strategy for price and delivery deadline.

It:

* supports buyer and seller roles;
* normalizes price and deadline values;
* preserves the supplied preference weights;
* uses a decreasing aspiration threshold;
* generates integer counteroffers that satisfy the current threshold;
* adds plugin registration and tests.

## Testing

The implementation passes:

* public seeds 42, 7, and 1337;
* Pareto non-domination and individual-rationality validators;
* an additional 100-seed robustness test.