Prepare coincurve 22.0.0 for Python 3.14 and free threading

[ofek]

Summary

• prepare coincurve 22.0.0 with wheels for CPython 3.10 through 3.14 and free-threaded CPython 3.14
• drop EOL Python 3.9 support and 32-bit Linux wheels
• support Python 3.15 and 3.15t source builds while intentionally excluding cp315 wheels until RC1
• remove limited-API compilation, use version-appropriate CFFI build requirements, and vendor the checked-in CFFI license only with the backend
• replace cross-emulated ARM wheel jobs with native runners and validate the exact 48-wheel plus one-sdist release inventory
• move benchmarks to a manual Ubuntu workflow, removing the unrelated macOS fastecdsa/GMP failure from release-gating CI

This addresses #212, #219, and #223. It incorporates the useful packaging and CI findings from #213, #214, and #224.

Validation

• formatting, mypy, actionlint, and documentation build
• randomized tests on Python 3.12 and Python 3.14t; all 51 tests pass under free threading
• exact-sdist builds and tests on locally available Python 3.15 and 3.15t prereleases
• isolated sdist/wheel build, Twine and wheel-content validation, clean wheel installation, and signing/verification smoke test
• vendored and non-vendored CFFI wheel-content checks
• completed 48-wheel CI matrix on native Linux, macOS, and Windows runners
• downloaded exact CI artifacts pass Twine, wheel-content, and release-inventory validation

Prerelease testing

CI tracks the latest Python 3.15 and 3.15t development interpreters available from setup-python. It verifies the interpreter minor version and GIL mode, then builds, installs, and tests the exact release sdist on Linux, macOS, and Windows. Python 3.15 wheels remain excluded until RC1.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.73%. Comparing base (4285045) to head (b700db3).
⚠️ Report is 16 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
+ Coverage   79.40%   79.73%   +0.33%     
==========================================
  Files          13       14       +1     
  Lines         738      755      +17     
  Branches       61       61              
==========================================
+ Hits          586      602      +16     
  Misses        105      105              
- Partials       47       48       +1     

Files with missing lines	Coverage Δ
src/coincurve/__init__.py	100.00% <100.00%> (ø)
src/coincurve/der.py	76.06% <100.00%> (ø)
tests/test_freethreading.py	100.00% <100.00%> (ø)

... and 14 files with indirect coverage changes

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