fix(stats): keep hidden Markov fitting alive on a single observation

Assisted-by: Qwen 3.8 Flash

stats/hmm.go

@@ -524,6 +524,15 @@ func hmmReestimate(model *HiddenMarkovModel, observations []int, gamma, xi [][]f
 		for j := range states {
 			den += transition[k*states+j]
 		}
+		if den == 0 {
+			// No transition evidence reached this row, the single-
+			// observation sequence being the case in point: dividing the
+			// zero fills the row with NaN the constructor refuses, and
+			// the sweep after it crashed on the nil model that refusal
+			// left. The row keeps its previous estimate instead.
+			copy(transition[k*states:(k+1)*states], model.Transition[k*states:(k+1)*states])
+			continue
+		}
 		for j := range states {
 			transition[k*states+j] = math.Max(transition[k*states+j]/den, hmmFloor)
 		}