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/expr/expr.go

@@ -1,5 +1,5 @@
 // Package expr provides the abstract Expression interface for all evaluatable
-// expression types in the lambda interpreter.
+// expression types in the lambda runtime.
 package expr
 
 import (