compiler: Write the complex arithmetic header atomically

[ggorman]

_complex_includes writes the extra complex arithmetic definitions a C++ language needs (def-complex) to complex_arith.h in the JIT cache directory, with open(..., 'w'), on every lowering of an Operator that uses a complex dtype. The directory is shared by every process compiling as the same user, and 'w' truncates the file before writing it, so with pytest -n auto one worker can truncate the header while another's compiler is reading it. The compile then fails on the first complex expression, e.g. no match for 'operator*' (operand types are 'int' and 'const std::complex<float>'); in DevitoPRO's CUDA backend the same race shows as user-defined literal operator not found for 1.0_if.

Changes:

• The header is keyed on its content (complex_arith_<sha1>.h), written to a private temporary file and renamed into place, and skipped when it already exists. A reader therefore sees either no file or a complete one, and two languages with different definitions no longer overwrite each other.
• test_complex_arith_header checks that two Operators share one header whose content is exactly def-complex.

Verification on main d7f53a3:

• 32 processes each JIT-compiling 30 distinct complex-valued language='CXX' Operators from one cache directory: main fails 7 of 960 compiles (twice, 7 each) with the error above; this patch 0 of 960, with a single complex_arith_<sha1>.h in the directory and no leftover temporaries.
• tests/test_dtypes.py: 47 passed, 5 skipped.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.23%. Comparing base (d7f53a3) to head (0f675e4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3071      +/-   ##
==========================================
- Coverage   84.23%   84.23%   -0.01%     
==========================================
  Files         258      258              
  Lines       57070    57089      +19     
  Branches     4874     4876       +2     
==========================================
+ Hits        48074    48087      +13     
- Misses       8169     8173       +4     
- Partials      827      829       +2     

Flag	Coverage Δ
pytest-gpu-aomp-amdgpuX	68.51% <25.00%> (+0.01%)	⬆️
pytest-gpu-gcc-	79.01% <100.00%> (+<0.01%)	⬆️
pytest-gpu-icx-	78.93% <100.00%> (+<0.01%)	⬆️
pytest-gpu-nvc-nvidiaX	69.09% <100.00%> (+<0.01%)	⬆️

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