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

cmd/lambda/lambda.go

@@ -6,9 +6,7 @@ import (
 	"git.maximhutz.com/max/lambda/internal/cli"
 	"git.maximhutz.com/max/lambda/internal/config"
 	"git.maximhutz.com/max/lambda/internal/engine"
-	"git.maximhutz.com/max/lambda/internal/explanation"
-	"git.maximhutz.com/max/lambda/internal/performance"
-	"git.maximhutz.com/max/lambda/internal/statistics"
+	"git.maximhutz.com/max/lambda/internal/plugins"
 	"git.maximhutz.com/max/lambda/pkg/convert"
 	"git.maximhutz.com/max/lambda/pkg/lambda"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
@@ -42,30 +40,23 @@ func main() {
 	// If the user selected to track CPU performance, attach a profiler to the
 	// process.
 	if options.Profile != "" {
-		profiler := performance.Track(options.Profile)
-		process.On("start", profiler.Start)
-		process.On("end", profiler.End)
+		plugins.NewPerformance(options.Profile, process)
 	}
 
 	// If the user selected to produce a step-by-step explanation, attach an
 	// observer here.
 	if options.Explanation {
-		explanation.Track(process)
+		plugins.NewExplanation(process)
 	}
 
 	// If the user opted to track statistics, attach a tracker here, too.
 	if options.Statistics {
-		statistics := statistics.Track()
-		process.On("start", statistics.Start)
-		process.On("step", statistics.Step)
-		process.On("end", statistics.End)
+		plugins.NewStatistics(process)
 	}
 
 	// If the user selected for verbose debug logs, attach a reduction tracker.
 	if options.Verbose {
-		process.On("step", func() {
-			logger.Info("reduction", "tree", lambda.Stringify(compiled))
-		})
+		plugins.NewLogs(logger, process)
 	}
 
 	process.Run()