Add support for Python 3.15 and upgrade to cibuildwheel 4.

CPython 3.15.0rc1 is out, and cibuildwheel 4.2.0 is the first release to
build cp315 wheels by default (rc1 is ABI-compatible with the final 3.15
release, so these wheels are safe to publish). NumPy 2.5.2 shipped cp315
and cp315t wheels on 2026-08-09, so 3.15 can be tested like any other
supported version rather than being treated as experimental.

 - Bump the cibuildwheel pin from >=3.0,<4 to >=4.2,<5.
 - Add cp315/cp315t wheel builds, mirroring the existing cp314 coverage:
   macOS arm64, Windows x86_64, and manylinux x86_64/aarch64.
 - Add Python 3.15 to the test and Address Sanitizer matrices, and pass
   allow-prereleases to actions/setup-python so it can resolve an rc.
 - Floor NumPy at 2.5.2 on 3.15 in test-requirements.txt, so pip can't
   quietly fall back to building an older NumPy from source there.
 - Add the 3.15 classifier to pyproject.toml and setup.py.

cibuildwheel 4.0 also made delvewheel the default repair step on
Windows. Pedalboard links statically, so this pins the Windows
repair-wheel-command back to empty to keep wheel contents unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ND3oQGgms3aXVsoBikUr93