pathfinder: also find ptxas in the nvidia/cu13/bin wheel layout

[rwgk]

Description

Triggered by #2739.

ptxas was not present in Pathfinder's supported binary utility registry. This PR adds it to SUPPORTED_BINARY_UTILITIES, enabling find_nvidia_binary_utility("ptxas") on Linux and Windows through the existing NVIDIA wheel, Conda, and CUDA Toolkit search pipeline.

For NVIDIA wheels, discovery prefers the CUDA 13 nvidia/cu13/bin location while retaining nvidia/cuda_nvcc/bin as a fallback. The CUDA 13 wheel-layout consistency test now covers ptxas as well.

The PR also adds the cuda-pathfinder 1.8.1 release notes, covering this change and the cuobjdump CUDA 13 wheel-layout fix from #2739, and registers 1.8.1 in the documentation version switcher.

Unrelated test dependency

This PR also adds pytest-run-parallel>=0.4.1 to Pathfinder's normal test dependencies. This is unrelated to ptxas discovery. Pathfinder already uses the plugin's thread_unsafe marker and thread_unsafe_fixtures configuration option, so installing the plugin prevents unknown-marker and unknown-config warnings in standard test environments.

Version 0.4.1 is the minimum compatible with the current pytest dependency; leaving the upper bound open allows normal ecosystem updates. The Pixi lockfile currently resolves pytest-run-parallel 0.10.0.

Testing

• Installed the test-cu13 dependency group and confirmed that Pathfinder resolves ptxas to the file supplied by nvidia-cuda-nvcc 13.3.73 under site-packages/nvidia/cu13/bin, rather than /usr/local/cuda-13.3/bin.
• Pathfinder test suite: 1610 passed, 5 skipped.
• Built the Pathfinder 1.8.1 documentation with Sphinx warnings treated as errors.
• pre-commit run --all-files.

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[rwgk]

/ok to test

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