surplus and fee calculations into reusable methods

[ashleychandy]

Description

Optimizes trade metric calculations by computing surplus once and reusing it, eliminating redundant calculations when surplus, fees, and breakdown are needed together.

Changes

• Added calculate_all_metrics() for single-pass calculation of surplus, fee, and breakdown
• Refactored surplus/fee methods to accept pre-computed values (surplus_to_ether(), fee_to_ether(), protocol_fees_from_surplus(), fee_breakdown_from_surplus())
• Introduced TradeMetrics struct to hold all metrics together

[gemini-code-assist]

Code Review

This pull request refactors trade metric calculations to support single-pass computation using pre-computed surplus values. It introduces the calculate_all_metrics method and several helper functions in the math module to derive ETH-denominated surplus and fees. No critical issues found; I have no feedback to provide.