Fix shadowing bug in rules engine.

cardsim/rules.go

@@ -141,7 +141,7 @@ func (r *RuleCollection[C]) performInsert(k *keyedRule[C]) {
 	r.rules[k.id] = k
 
 	s := r.byStep[k.Step()]
-	if s == nil {
+	if len(s) == 0 {
 		r.steps = nil
 	}
 	s = append(s, k.id)
@@ -326,7 +326,7 @@ func (r *RuleCollection[C]) Run(p *Player[C]) error {
 	steps := r.steps
 	if steps == nil {
 		// Step set changed, recalculate.
-		steps := make([]int, 0, len(r.byStep))
+		steps = make([]int, 0, len(r.byStep))
 		for step := range r.byStep {
 			steps = append(steps, step)
 		}