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
+	}
+}