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

internal/engine/engine.go

@@ -12,7 +12,7 @@ import (
 type Engine struct {
 	Config     *config.Config
 	Expression *lambda.Expression
-	emitter.Emitter
+	emitter.BaseEmitter[Event]
 }
 
 // Create a new engine, given an unreduced λ-expression.
@@ -22,11 +22,11 @@ func New(config *config.Config, expression *lambda.Expression) *Engine {
 
 // Begin the reduction process.
 func (e Engine) Run() {
-	e.Emit("start")
+	e.Emit(StartEvent)
 
 	lambda.ReduceAll(e.Expression, func() {
-		e.Emit("step")
+		e.Emit(StepEvent)
 	})
 
-	e.Emit("end")
+	e.Emit(StopEvent)
 }