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>

.gitea/ISSUE_TEMPLATE/bug.md

@@ -1,6 +1,6 @@
 ---
 name: "Bug Report"
-about: "Report a bug or unexpected behavior in the lambda interpreter."
+about: "Report a bug or unexpected behavior in the lambda runtime."
 title: "fix: "
 ref: "main"
 assignees: []