A multi-attribute negotiation extreme strategy

[anirjoshi]

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.