Support PEP 803 abi3t free-threaded Stable ABI

[timkpaine]

Description

Add support for PEP 803 free-threaded CPython 3.15+ extension modules and wheels built against the abi3t Stable ABI. The changes also make fixture test execution more reliable on Windows.

This repository-owned branch replaces #81 because its source fork does not allow maintainer edits. The original commits and authorship are preserved.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup
• CI / build configuration
• Other (describe below)

Checklist

• Linting passes (make lint)
• Tests pass (make test)
• New tests added for new functionality
• Documentation updated (if applicable)
• Changelog / version bump (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 82.14286% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.99%. Comparing base (f63bf62) to head (eafbfc6).

Files with missing lines	Patch %	Lines
hatch_rs/structs.py	75.67%	9 Missing ⚠️
hatch_rs/tests/test_projects.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
- Coverage   87.16%   86.99%   -0.18%     
==========================================
  Files          12       12              
  Lines        1621     1669      +48     
==========================================
+ Hits         1413     1452      +39     
- Misses        208      217       +9     

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