Add Python 3.15 support to package and CI

[mcdigman]

Summary

• Declare CPython 3.15 support in the package classifiers and contributor documentation.
• Add 3.15 to the normal test matrix, use it for the fast test job, and exercise it in the build and install jobs.
• Refresh uv.lock to Numba 0.68.0, llvmlite 0.50.0, and a NumPy release with CPython 3.15 wheels.

requires-python = ">=3.10" already includes 3.15, so it does not need a change. The low runtime dependency floors remain in place for older supported Python versions.

Verification

• Python 3.15.0rc2: 189 tests passed with uv sync --locked --extra dev.
• Python 3.15.0rc2: 189 tests passed with the CI-style highest-resolution install.
• Python 3.14.6: 189 tests passed with the refreshed lockfile.
• Built sdist and wheel using Python 3.15.0rc2; confirmed the wheel metadata contains the 3.15 classifier.
• uv lock --check --offline, git diff --check, and commit hooks passed.

Hosted GitHub Actions checks will validate the Linux matrix on this PR.