refactor: rename interpreter to runtime and use receiver methods

- Rename pkg/interpreter to pkg/runtime
- Move ReduceOnce to new pkg/normalorder package
- Convert standalone functions (Substitute, Rename, GetFree, IsFree)
  to receiver methods on concrete expression types
- Change Set from pointer receivers to value receivers
- Update all references from interpreter to runtime terminology

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/plugins/statistics.go

@@ -6,7 +6,7 @@ import (
 	"time"
 
 	"git.maximhutz.com/max/lambda/internal/statistics"
-	"git.maximhutz.com/max/lambda/pkg/interpreter"
+	"git.maximhutz.com/max/lambda/pkg/runtime"
 )
 
 // An observer, to track reduction performance.
@@ -16,11 +16,11 @@ type Statistics struct {
 }
 
 // Create a new reduction performance Statistics.
-func NewStatistics(r interpreter.Interpreter) *Statistics {
+func NewStatistics(r runtime.Runtime) *Statistics {
 	plugin := &Statistics{}
-	r.On(interpreter.StartEvent, plugin.Start)
-	r.On(interpreter.StepEvent, plugin.Step)
-	r.On(interpreter.StopEvent, plugin.Stop)
+	r.On(runtime.StartEvent, plugin.Start)
+	r.On(runtime.StepEvent, plugin.Step)
+	r.On(runtime.StopEvent, plugin.Stop)
 
 	return plugin
 }