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>

pkg/normalorder/reduce_once.go

@@ -0,0 +1,34 @@
+package normalorder
+
+import "git.maximhutz.com/max/lambda/pkg/lambda"
+
+func ReduceOnce(e lambda.Expression) (lambda.Expression, bool) {
+	switch e := e.(type) {
+	case lambda.Abstraction:
+		body, reduced := ReduceOnce(e.Body())
+		if reduced {
+			return lambda.NewAbstraction(e.Parameter(), body), true
+		}
+		return e, false
+
+	case lambda.Application:
+		if fn, fnOk := e.Abstraction().(lambda.Abstraction); fnOk {
+			return fn.Body().Substitute(fn.Parameter(), e.Argument()), true
+		}
+
+		abs, reduced := ReduceOnce(e.Abstraction())
+		if reduced {
+			return lambda.NewApplication(abs, e.Argument()), true
+		}
+
+		arg, reduced := ReduceOnce(e.Argument())
+		if reduced {
+			return lambda.NewApplication(e.Abstraction(), arg), true
+		}
+
+		return e, false
+
+	default:
+		return e, false
+	}
+}

pkg/normalorder/runtime.go

@@ -0,0 +1,46 @@
+package normalorder
+
+import (
+	"git.maximhutz.com/max/lambda/pkg/emitter"
+	"git.maximhutz.com/max/lambda/pkg/expr"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
+	"git.maximhutz.com/max/lambda/pkg/runtime"
+)
+
+// NormalOrderReducer implements normal order (leftmost-outermost) reduction
+// for lambda calculus expressions.
+type Runtime struct {
+	emitter.BaseEmitter[runtime.Event]
+	expression lambda.Expression
+}
+
+// NewNormalOrderReducer creates a new normal order reducer.
+func NewRuntime(expression lambda.Expression) *Runtime {
+	return &Runtime{
+		BaseEmitter: *emitter.New[runtime.Event](),
+		expression:  expression,
+	}
+}
+
+// Expression returns the current expression state.
+func (r *Runtime) Expression() expr.Expression {
+	return r.expression
+}
+
+func (r *Runtime) Step() bool {
+	result, done := ReduceOnce(r.expression)
+	r.expression = result
+	return !done
+}
+
+// Reduce performs normal order reduction on a lambda expression.
+// The expression must be a lambda.Expression; other types are returned unchanged.
+func (r *Runtime) Run() {
+	r.Emit(runtime.StartEvent)
+
+	for !r.Step() {
+		r.Emit(runtime.StepEvent)
+	}
+
+	r.Emit(runtime.StopEvent)
+}