feat: implement goto/label support

[esrrhs]

Summary

实现 Lua goto/label 语法支持。

语义分析（semantic_analysis.cpp）

• 移除 CheckGotoOrLabel() 的拦截抛异常逻辑
• 新增 ValidateGotoInBlock()：递归验证 goto 合法性
• 验证规则：
  1. goto 目标 label 必须在可见作用域内
  2. goto 不能跳过局部变量声明（Lua 5.2+ 规则）
  3. label 在嵌套 block 内也可见（作用域传递）
  4. 函数是独立作用域，内部 label/goto 不影响外部

C 代码生成（c_gen.cpp）

• 新增 CompileStmtGoto()：生成 goto flua_L_<name>;
• 新增 CompileStmtLabel()：生成 flua_L_<name>: ;

AST（syntax_tree.h）

• SyntaxTreeGoto 新增 GetLabel() 方法

测试

• 新增 4 个测试：forward skip、backward loop、label-only、invalid goto
• 移除 2 个旧的异常测试（goto/label 不再被拒绝）

全部 715 个测试通过

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.77%. Comparing base (e28a39a) to head (44f546b).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
+ Coverage   96.69%   96.77%   +0.07%     
==========================================
  Files          67       67              
  Lines       14008    14118     +110     
==========================================
+ Hits        13545    13662     +117     
+ Misses        463      456       -7     

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