Smallest y with weighted CDF >= p. Uses a small
numerical tolerance on the comparison so that a p landing exactly
on a floating-point rounding boundary (common with uniform weights, e.g.
1/n) picks the intended order statistic instead of flipping based
on last-bit differences in summation across platforms.
wquant(y_vec, w_vec, probs)numeric vector, same length as probs.
numeric vector of values.
numeric vector of weights, same length as y_vec.
numeric vector of probabilities in [0, 1].