Vega Surface

Vega measures how much an option's price changes per 1% move in implied volatility.

**Your task:** Implement vega_surface(S, K_list, T_list, r, sigma) that returns a 2D list (matrix) of vega values, where rows correspond to strikes K and columns to expiries T.

Each cell should equal: S · N'(d₁) · √T / 100

where N'(x) is the standard normal PDF and d₁ = [ln(S/K) + (r + σ²/2)·T] / (σ·√T).