fix: exact return counts and scope-aware math-spec eligibility

[esrrhs]

Summary

• 尾返回按「前缀表达式个数 + 被调返回数」计算；return ... / return x, ... 记为未知。只有所有路径返回数一致时才保留精确值，否则调用点按多返回值展开。
• return f() 按词法作用域解析被调。形参、局部变量，以及与文件级函数同名的嵌套 local function，不再套用文件级函数的返回数或数学特化。没有同名文件级函数、且嵌套函数精确返回 1 个值时，外层仍然可以特化。
• 同名嵌套函数不再生成把 FlMakeMulti 放进 int64_t 特化函数的非法 C。C 代码生成和解释器都会调用局部绑定，而不是同名的文件级函数。

Test plan

• infer.test_jitbug_vararg_passthrough：纯 return ... 在调用点展开
• infer.test_jitbug_return_exact：return a, f() 精确计数，路径不一致时不特化
• infer.test_jitbug_return_scope：同名嵌套函数 / 形参不特化，无同名嵌套函数仍可特化，运行结果为 62
• 相关既有 infer 测试（test_spec_fib、test_spec_nested_call、test_count_loop 等）通过

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Codecov Report

❌ Patch coverage is 97.52650% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.86%. Comparing base (4e396e8) to head (0374b12).

Files with missing lines	Patch %	Lines
src/compile/semantic_analysis.cpp	97.59%	6 Missing ⚠️
src/compile/c_gen.cpp	96.77%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
- Coverage   87.95%   87.86%   -0.09%     
==========================================
  Files         122      122              
  Lines       24482    24657     +175     
==========================================
+ Hits        21533    21666     +133     
- Misses       2949     2991      +42     

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