fix(stats): refuse a zero-probability sequence in Viterbi

Assisted-by: GLM 5.3 Flash

stats/hmm.go

@@ -354,6 +354,12 @@ func (m *HiddenMarkovModel) Viterbi(observations []int) (states []int, logProbab
 			arg = k
 		}
 	}
+	if math.IsInf(best, -1) {
+		// Every path has probability zero, the structural zero an
+		// emission or transition carries: the same refusal Forward makes,
+		// not a meaningless path beside a −Inf log probability.
+		return nil, 0, base.Errf("%s: the observation sequence has probability zero under the model", name)
+	}
 	path := make([]int, len(observations))
 	path[len(observations)-1] = arg
 	for t := len(observations) - 1; t > 0; t-- {