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/debug.go

@@ -0,0 +1,25 @@
+package plugins
+
+import (
+	"log/slog"
+
+	"git.maximhutz.com/max/lambda/internal/engine"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
+)
+
+type Logs struct {
+	logger  *slog.Logger
+	process *engine.Engine
+}
+
+func NewLogs(logger *slog.Logger, process *engine.Engine) *Logs {
+	plugin := &Logs{logger, process}
+	process.On(engine.StopEvent, plugin.Step)
+
+	return plugin
+}
+
+func (t *Logs) Step() {
+	stringified := lambda.Stringify(*t.process.Expression)
+	t.logger.Info("reduction", "tree", stringified)
+}