fix(analyze): compare arms on censored runs, not on flattened step counts

stepTimes threw the censoring flag away and handed the rank-sum a plain
number per run, so a run the wall clock stopped at step 12 was ranked as one
that violated at step 12. That was defensible while every clean run sat at
the budget, the largest value any run could take, and it stopped being
defensible when a clean run started being censored where it stopped.

Twenty runs clean at step 12 against twenty violations at step 100 read a12
0.000 and p 4.683e-10 from the rank-sum, in the same report as a log-rank
reading p 1.0000. The pairwise comparison is now the Gehan test over the
observations themselves, and the report says how many run pairs censoring
left with no order between them, which is how much of the effect size is the
null value rather than an observation.

cmd/internal-tools/analyze/analysis.go

@@ -45,11 +45,15 @@ type pairwiseResult struct {
 	Second     string  `json:"second"`
 	FirstSize  int     `json:"first_size"`
 	SecondSize int     `json:"second_size"`
-	Statistic  float64 `json:"mann_whitney_u"`
+	Statistic  float64 `json:"u"`
 	A12        float64 `json:"a12"`
+	// Unordered is how many of the run pairs behind U and A12 have no order
+	// between them, because they were tied or because censoring stopped one run
+	// before the other violated. Each of them counts as half, so it is also how
+	// much of the effect size is the null value rather than an observation.
+	Unordered  int     `json:"unordered_pairs"`
 	PValue     float64 `json:"p_value"`
 	HolmPValue float64 `json:"holm_p_value"`
-	Exact      bool    `json:"exact"`
 }
 
 type analysis struct {
@@ -197,7 +201,7 @@ func comparePairs(arms []arm) []pairwiseResult {
 	var pairs []pairwiseResult
 	for first := 0; first < len(arms); first++ {
 		for second := first + 1; second < len(arms); second++ {
-			test := rankSum(arms[first].stepTimes(), arms[second].stepTimes())
+			test := gehanTest(arms[first].observations(), arms[second].observations())
 			pairs = append(pairs, pairwiseResult{
 				First:      arms[first].Name,
 				Second:     arms[second].Name,
@@ -205,9 +209,9 @@ func comparePairs(arms []arm) []pairwiseResult {
 				SecondSize: test.SecondSize,
 				Statistic:  test.Statistic,
 				A12:        test.A12,
+				Unordered:  test.Unordered,
 				PValue:     test.PValue,
 				HolmPValue: math.NaN(),
-				Exact:      test.Exact,
 			})
 		}
 	}