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/explanation.go

@@ -5,19 +5,19 @@ package plugins
 import (
 	"fmt"
 
-	"git.maximhutz.com/max/lambda/pkg/interpreter"
+	"git.maximhutz.com/max/lambda/pkg/runtime"
 )
 
 // Track the reductions made by a reduction process.
 type Explanation struct {
-	reducer interpreter.Interpreter
+	reducer runtime.Runtime
 }
 
 // Attaches a new explanation tracker to a reducer.
-func NewExplanation(r interpreter.Interpreter) *Explanation {
+func NewExplanation(r runtime.Runtime) *Explanation {
 	plugin := &Explanation{reducer: r}
-	r.On(interpreter.StartEvent, plugin.Start)
-	r.On(interpreter.StepEvent, plugin.Step)
+	r.On(runtime.StartEvent, plugin.Start)
+	r.On(runtime.StepEvent, plugin.Step)
 
 	return plugin
 }