== erf 两版本最大差（|x|<=8，4001 点） ==
  max|erf_as - math.erf| = 1.3931301849623434e-07
== 向量化 vs 字面实现 ==
  max|diff| = 1.1102230246251565e-16
  vec = [0.8024122140436516  0.9750739156075907  0.
 0.                  0.20991871369486403 0.46450022345727304]
  lit = [0.8024122140436516 0.9750739156075906 0.
 0.                 0.2099187136948641 0.464500223457273 ]
== L 公式 ==
  n=2 L=1
  n=3 L=1
  n=50 L=3
  n=120 L=4
  n=242 L=4
  n=1212 L=6
  n=2424 L=8
== 用例 B: group_returns[d][t]=d, n=50 ==
  math.erf 版 = 0.0
  A&S 版     = 0.0
== 用例 C: 全 5.0, n=50 ==
  math.erf 版 = 0.0
  A&S 版     = 0.0
== SPEC 用例 1/2 的两种可能读法 ==
  单日(10x1)      = nan
  组间阶梯(10x10) = 0.0
  反向阶梯(10x10) = 0.0
== 用例 A: seed 20240501, 10x120 ==
  math.erf 版 = 0.20324340391338103
  A&S 版     = np.float64(0.20324338436656386)
  差 = 1.954681716487272e-08
== 空值参考分布 20000 次, T=2424 ==
    5% = -0.1272
   25% = -0.0015
   50% = +0.0897
   75% = +0.1852
   90% = +0.2652
   95% = +0.3140
   99% = +0.3995
  sd = 0.1336
  mean = 0.0919
