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)
+}

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))
+}

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()
+}

internal/plugins/statistics.go

@@ -0,0 +1,44 @@
+package plugins
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"git.maximhutz.com/max/lambda/internal/engine"
+	"git.maximhutz.com/max/lambda/internal/statistics"
+)
+
+// An observer, to track reduction performance.
+type Statistics struct {
+	start time.Time
+	steps uint64
+}
+
+// Create a new reduction performance Statistics.
+func NewStatistics(process *engine.Engine) *Statistics {
+	plugin := &Statistics{}
+	process.On(engine.StartEvent, plugin.Start)
+	process.On(engine.StepEvent, plugin.Step)
+	process.On(engine.StopEvent, plugin.Step)
+
+	return plugin
+}
+
+func (t *Statistics) Start() {
+	t.start = time.Now()
+	t.steps = 0
+}
+
+func (t *Statistics) Step() {
+	t.steps++
+}
+
+func (t *Statistics) Stop() {
+	results := statistics.Results{
+		StepsTaken:  t.steps,
+		TimeElapsed: uint64(time.Since(t.start).Milliseconds()),
+	}
+
+	fmt.Fprint(os.Stderr, results.String())
+}