fix(analyze): censor a clean run at the steps it ran, and refuse mismatched budgets

A run stops at whichever comes first, the step budget or --duration, so
a clean run that reached the wall clock exited with fewer steps than the
budget and was still credited with the whole of it. The model arm pays a
network call and a screenshot per step, so it reaches the wall sooner and
was handed exposure it never had.

Nothing checked that two arms shared a budget either. Thirty identical
clean runs under budgets of 400 and 100 read a12 0.000 and p 1.685e-14
from the rank-sum while the log-rank in the same report read p 1.0000.
groupArms already refused this within one arm.

The claims the old convention left in comments and report lines are
corrected rather than left standing beside the new behaviour.

cmd/internal-tools/analyze/end_to_end_test.go

@@ -90,7 +90,7 @@ func TestRun_EndToEndOverFixtureCampaignDirectories(t *testing.T) {
 
 	text := stdout.String()
 	for _, fragment := range []string{
-		"steps to first violation, right-censored at the step budget",
+		"steps to first violation, right-censored at the last step a clean run reached",
 		"log-rank across 2 arms",
 		"pairwise wilcoxon rank-sum",
 		"llm vs seeded",