Add Brent's method optimization algorithm (#328)

Implements Brent's method for scalar function minimization, combining
golden section search with parabolic interpolation for faster convergence.

Key features:
- Pure defn implementation (JIT/GPU compatible)
- ~3-5x fewer function evaluations than Golden Section
- Same API pattern as GoldenSection.minimize/4

Also updates notebooks/optimize.livemd to present Brent as the
recommended method with performance comparisons.

Author: nyo16