ci(wheel-build): tighten and speed up wheel build workflow

[ryankert01]

What

Improvements to the wheel-build.yml workflow:

• paths filters on push/pull_request: wheel builds only run when qumat/**, qdp/**, pyproject.toml, uv.lock, or the workflow itself changes. Website/docs-only PRs no longer trigger the expensive CUDA build. (.asf.yaml defines no required status checks, so skipped runs don't block PRs.)
• concurrency group: pushing new commits to a PR cancels the superseded run; main/workflow_dispatch runs are never cancelled.
• Single build job instead of a 3-version matrix: one maturin invocation builds all three interpreters (-i python3.10 -i python3.11 -i python3.12, same as the release workflow), so the CUDA toolkit and gcc-toolset are installed once instead of three times.
• Removed the silent QDP_NO_CUDA fallback: previously, if the NVIDIA repo install failed, CI built a no-CUDA smoke wheel and uploaded it under the same artifact name with a green check. Now the job fails instead — green always means real kernels were compiled. Inspection of all recent runs with retained logs showed the fallback path has never actually fired; the CUDA repo install succeeds consistently.
• Artifact validation before upload:
  • twine check on all distributions in both jobs
  • import qumat smoke test for the pure-Python wheel
  • each qumat-qdp wheel is installed into a matching per-version venv and the test asserts the Rust extension actually loads (get_qdp() is not None), guarding against the graceful-degradation path in qumat_qdp/_backend.py masking a broken extension. This is safe on GPU-less runners because cudarc loads CUDA libraries lazily at runtime.
• CUDA version defined once at the top of the build script instead of hard-coded in five places.

Notes

• The qdp artifact is now a single qumat-qdp-wheels artifact containing all three wheels, instead of three qumat-qdp-cpX.Y artifacts.
• release.yml is intentionally untouched.

[Copilot]

Pull request overview

This PR updates the wheel-build.yml GitHub Actions workflow to reduce unnecessary/expensive wheel build runs and make the wheel artifacts more trustworthy by adding validation and removing a silent no-CUDA fallback.

Changes:

• Adds paths filters and PR-only run cancellation via concurrency to reduce redundant CI runs.
• Collapses the CUDA wheel build from a Python-version matrix into a single maturin build that targets 3.10/3.11/3.12 in one job.
• Adds artifact validation (twine checks + install/import smoke tests) and removes the “build without CUDA” fallback so green builds imply real CUDA compilation.

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