[shaanmajid]

This adds Python 3.15 and 3.15t to the test matrix on Linux, macOS and Windows, and builds cp315t wheels alongside the existing cp38-abi3 and cp314t wheels. Also adds the 3.15 classifier in pyproject.toml.

Notes:

• The == '3.14t' conditions in wheel-builder.yml now use endsWith(..., 't'), so the free-threaded steps apply to both 3.14t and 3.15t.
• The macOS free-threaded installer choices file names the 3.14 framework package, so the workflow substitutes the matrix version before passing it to installer.
• setuptools does not yet support abi3t.
  • In the future, a single abi3t wheel could replace per-version free-threaded wheels (for 3.15t and subsequent versions) once setuptools can tag them; I'm happy to follow up with that once setuptools support is released if you'd like.
  • Alternatively, there is a documented workaround, but I've held off on that for now