refactor: replace string-based emitter with type-safe generic event system

Refactors the event emitter system from string-based messages to a
type-safe generic implementation using typed events. Consolidates
separate tracker packages into a unified plugins architecture.

Changes:
- Replace Emitter with BaseEmitter[E comparable] using generics
- Add Event type with StartEvent, StepEvent, and StopEvent constants
- Create Listener[E] interface with BaseListener implementation
- Consolidate explanation, performance, and statistics trackers into
  internal/plugins package
- Simplify main CLI by using plugin constructors instead of manual
  event subscription
- Add Items() iterator method to Set for idiomatic range loops

pkg/set/set.go

@@ -1,5 +1,7 @@
 package set
 
+import "iter"
+
 type Set[T comparable] map[T]bool
 
 func (s *Set[T]) Add(items ...T) {
@@ -34,6 +36,16 @@ func (s Set[T]) ToList() []T {
 	return list
 }
 
+func (s Set[T]) Items() iter.Seq[T] {
+	return func(yield func(T) bool) {
+		for item := range s {
+			if !yield(item) {
+				return
+			}
+		}
+	}
+}
+
 func New[T comparable](items ...T) *Set[T] {
 	result := &Set[T]{}