v0.4.0

Changed (breaking)

• functionals.auroc no longer accepts a tau parameter. The
  sigmoid-smoothed surrogate is removed; the closed form now attributes
  the exact Mann-Whitney AUROC via perturbation evaluation (below).

Added

• Perturbation evaluation for non-differentiable functionals.
  Functional gained a differentiable flag (default True). Rank
  statistics and other piecewise-constant functionals are attributed by
  evaluating the exact functional on each removal's linearized value
  change, score[j] = F(v + dv_j) - F(v), computed in memory-bounded
  column blocks. This preserves the quantized structure of the estimand
  (removals that swap no pair score exactly zero) and agrees with exact
  leave-one-out refitting at r > 0.99 in the test suite. For smooth
  functionals the perturbation and chain-rule paths agree to first
  order; the estimator API is unchanged.
• README code blocks are executed as part of the test suite.