feat: add GeometricMean coefficient combine rule

[HueCodes]

Closes #685

Fresh implementation since #826 has merge conflicts.

Adds CoefficientCombineRule::GeometricMean which computes sqrt(coeff1 * coeff2).

This provides a more physically realistic way to combine friction/restitution coefficients:

• For values 0.1 and 0.9: Average gives 0.5, GeometricMean gives 0.3
• If one value is 0 (perfectly inelastic), result is 0 (not a bounce)
• Equal values unchanged: sqrt(0.8 × 0.8) = 0.8

Uses .abs() before sqrt to handle negative coefficients defensively (consistent with Min rule).

Includes unit tests for basic usage, edge cases (zero, negative, same values), priority, and ordering.