The report now runs a chi-square sample-ratio-mismatch test of the arm counts
against the expected 50/50 split, on both the raw assigned counts and the matured
subset (to catch differential attrition), flagging a low p as an imbalance to
investigate before trusting results. Extract a shared Normal::cdf so the SRM and
Welch tests use one erf implementation.
Víctor Falcón