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/performance.go

@@ -0,0 +1,59 @@
+// Package "performance" provides a tracker to observer CPU performance during
+// execution.
+package plugins
+
+import (
+	"os"
+	"path/filepath"
+	"runtime/pprof"
+
+	"git.maximhutz.com/max/lambda/internal/engine"
+)
+
+// Observes a reduction process, and publishes a CPU performance profile on
+// completion.
+type Performance struct {
+	File        string
+	filePointer *os.File
+	Error       error
+}
+
+// Create a performance tracker that outputs a profile to "file".
+func NewPerformance(file string, process *engine.Engine) *Performance {
+	plugin := &Performance{File: file}
+	process.On(engine.StartEvent, plugin.Start)
+	process.On(engine.StopEvent, plugin.Stop)
+
+	return plugin
+}
+
+// Begin profiling.
+func (t *Performance) Start() {
+	var absPath string
+
+	absPath, t.Error = filepath.Abs(t.File)
+	if t.Error != nil {
+		return
+	}
+
+	t.Error = os.MkdirAll(filepath.Dir(absPath), 0777)
+	if t.Error != nil {
+		return
+	}
+
+	t.filePointer, t.Error = os.Create(absPath)
+	if t.Error != nil {
+		return
+	}
+
+	t.Error = pprof.StartCPUProfile(t.filePointer)
+	if t.Error != nil {
+		return
+	}
+}
+
+// Stop profiling.
+func (t *Performance) Stop() {
+	pprof.StopCPUProfile()
+	t.filePointer.Close()
+}