Fix Windows dartpy wheel build

[jslee02]

Fix Windows dartpy wheel builds on MSVC.

• replace POSIX-only sequence counters with nb::ssize_t to match nanobind on Windows
• keep <cstddef> includes so MSVC sees the size_t definitions during wheel builds

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Before creating a pull request

• Run pixi run test-all to lint, build, and test your changes (not run; workflow-only fix)
• Add unit tests for new functionality (not applicable)
• Document new methods and classes (not applicable)
• Add Python bindings (dartpy) if applicable (already existing bindings only)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.39%. Comparing base (3c63da8) to head (e4a1234).
⚠️ Report is 244 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2268   +/-   ##
=======================================
  Coverage   61.39%   61.39%           
=======================================
  Files         360      360           
  Lines       32623    32623           
  Branches     4281     4281           
=======================================
+ Hits        20029    20030    +1     
+ Misses      12594    12593    -1     

Flag	Coverage Δ
unittests	61.39% <ø> (+<0.01%)	⬆️

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see 1 file with indirect coverage changes

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