feat(verifier): expose the step a snapshot was observed at

It lags the runner's current step whenever a transitional tree caused an
observation to be skipped, which is exactly when the model is shown an older
screen than the step it is choosing for.

Claude-Session: https://claude.ai/code/session_01A5KmftdEJ49A9z5mF5ESrX

internal/verifier/llm.go

@@ -65,6 +65,13 @@ func (v *Verifier) Screenshot() []byte {
 	return v.lastScreenshot
 }
 
+// SnapshotStep returns the step index of the most recent PushSnapshot. It lags
+// the runner's current step whenever an observation was skipped (a transitional
+// tree), which is exactly when Screenshot returns an older step's image.
+func (v *Verifier) SnapshotStep() int {
+	return v.stepIndex
+}
+
 // CurrentScreen returns the screen id of the most recent snapshot's first
 // element, matching the runner's own screen labeling. Empty when no tree is
 // loaded.