feat: add math.atan2, math.copysign, string.charpattern, loadfile

[esrrhs]

Summary

• Add math.atan2(y, x): two-argument arctangent
• Add math.copysign(x, y): copy sign of y to magnitude of x
• Add string.charpattern: compile-time constant pattern "[^%z]" (matches any character except null)
• Add loadfile(filename): load Lua source from file, compile directly so top-level functions become global, return closure that executes the chunk when called

Implementation

• math.atan2 / math.copysign: registered as native functions in native_math.cpp
• string.charpattern: compile-time constant interception in c_gen.cpp (same pattern as math.pi)
• loadfile: reads file, compiles via CompileString (so globals register properly), returns a closure with special marker \x1bLoadFile that FlEvalLoadClosure recognizes to call __fakelua_init directly

Test Coverage

• 4 new test cases (each runs under both JIT_TCC and JIT_GCC):
  • test_math.test_math_atan2
  • test_math.test_math_copysign
  • test_string.test_string_charpattern
  • test_string.test_string_loadfile
• All 33 math/string/basic tests pass

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 97.53086% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.80%. Comparing base (b1313f1) to head (14bdca0).

Files with missing lines	Patch %	Lines
src/native/native_string.cpp	86.66%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
+ Coverage   90.77%   90.80%   +0.02%     
==========================================
  Files          76       76              
  Lines       17439    17511      +72     
==========================================
+ Hits        15831    15901      +70     
- Misses       1608     1610       +2     

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