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/plugins/explanation.go

@@ -0,0 +1,32 @@
+// Package "explanation" provides a observer to gather the reasoning during the
+// reduction, and present a thorough explanation to the user for each step.
+package plugins
+
+import (
+	"fmt"
+
+	"git.maximhutz.com/max/lambda/internal/engine"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
+)
+
+// Track the reductions made by a reduction proess.
+type Explanation struct {
+	process *engine.Engine
+}
+
+// Attaches a new explanation tracker to a process.
+func NewExplanation(process *engine.Engine) *Explanation {
+	plugin := &Explanation{process: process}
+	process.On(engine.StartEvent, plugin.Start)
+	process.On(engine.StepEvent, plugin.Step)
+
+	return plugin
+}
+
+func (t *Explanation) Start() {
+	fmt.Println(lambda.Stringify(*t.process.Expression))
+}
+
+func (t *Explanation) Step() {
+	fmt.Println("    =", lambda.Stringify(*t.process.Expression))
+}