feat: 适配 LuaJIT 2.1 (luajit_2.1 分支)

- lib/lua 替换为完整 LuaJIT 2.1 仓库(Makefile+src+dynasm, 官方结构)
- 构建系统改造: lib/lua/CMakeLists.txt 改为调用 LuaJIT 自带 make
  (BUILDMODE=static CFLAGS=-fPIC) 产出 libluajit.a; 顶层链接该静态库
  并加 -Wl,-E 导出符号(静态链 LuaJIT 必需)
- 补充 lua.hpp 转发头兼容桥接层 'lua/lua.hpp' include 路径
- 桥接层沿用 5.1 方案: Registry 存 iopath / luaL_tolstring 兜底 /
  typed interop 无 integer 子类型(LUA_VERSION_NUM=501)
- [关键修复] lua_remove 栈越界 bug(各版本共有, LuaJIT 下显性崩溃):
  * unload_lib: ref 记录的是 gettop+1, 实际移除应用 ref-1
  * clean(): 原逐个 remove 循环恒越界且多次移除后索引偏移,
    属死代码(末尾 settop(L,0) 已清栈), 直接删除
  该 bug 在 Lua 5.4 中被隐式容忍, 在 LuaJIT 的 GC finalizer 中
  表现为 lj_gc_finalize_cdata 段错误
- 测试跨版本断言同步(自 5.1.5)
- 验证: GTest 44/44 通过

lib/lua/README

@@ -1,6 +1,16 @@
+README for LuaJIT 2.1
+---------------------
 
-This is Lua 5.4.8, released on 21 May 2025.
+LuaJIT is a Just-In-Time (JIT) compiler for the Lua programming language.
 
-For installation instructions, license details, and
-further information about Lua, see doc/readme.html.
+Project Homepage: https://luajit.org/
+
+LuaJIT is Copyright (C) 2005-2026 Mike Pall.
+LuaJIT is free software, released under the MIT license.
+See full Copyright Notice in the COPYRIGHT file or in luajit.h.
+
+Documentation for LuaJIT is available in HTML format.
+Please point your favorite browser to:
+
+ doc/luajit.html