Improve dart8 shared builds and CI stability

[jslee02]

• honor BUILD_SHARED_LIBS for dart8 and ensure dart8 tests prepend the DLL path so shared builds work on Windows
• guard serializer registration to avoid duplicate dart8 registrations
• cap default build parallelism to avoid posix_spawn resource errors on Linux runners
• avoid re-installing pixi on Windows when already present and skip the install step to keep jobs within limits

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Before creating a pull request

• Run pixi run test-all to lint, build, and test your changes (relied on GitHub CI)
• Add unit tests for new functionality (not applicable)
• Document new methods and classes (not applicable)
• Add Python bindings (dartpy) if applicable (not applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.26%. Comparing base (7fcbfc6) to head (feb20b6).
⚠️ Report is 289 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2228      +/-   ##
==========================================
- Coverage   61.27%   61.26%   -0.01%     
==========================================
  Files         352      352              
  Lines       31547    31547              
  Branches     4084     4083       -1     
==========================================
- Hits        19331    19328       -3     
- Misses      12216    12219       +3     

Flag	Coverage Δ
unittests	61.26% <ø> (-0.01%)	⬇️

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see 1 file with indirect coverage changes

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