cuda.core.system: Add basic Nvlink and Utilization support

[mdboom]

These APIs are needed by rapidsai/jupterlab-nvdashboard and rapidsai/rapids-cli

[github-actions]

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[rwgk]

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Manually verified.

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Medium: Invalid NVLink indices are accepted and fail late

Device.nvlink() currently accepts negative or out-of-range link indices and
returns NvlinkInfo without validating them first. That differs from existing
indexed accessors such as Device.fan(), which validate eagerly. In practice,
device.nvlink(-1) constructs successfully and only fails later when a
property such as .version is accessed, which turns a basic argument error
into a delayed runtime failure.

Relevant paths:

• cuda_core/cuda/core/system/_device.pyx:585
• cuda_core/cuda/core/system/_device.pyx:683
• cuda_core/cuda/core/system/_nvlink.pxi

Low: NvlinkInfo.version documents a non-existent return type

The public enum exported by cuda.core.system is NvlinkVersion, and the API
index plus tests use that spelling, but NvlinkInfo.version is annotated and
documented as NvLinkVersion. That leaks a wrong type name into the generated
help/doc output and points users at a symbol that does not exist.

Relevant paths:

• cuda_core/cuda/core/system/_nvlink.pxi:21
• cuda_core/docs/source/api.rst:225
• cuda_core/tests/system/test_system_device.py:747

Low: NvlinkInfo.state has no direct test coverage

The new test_nvlink() checks construction of NvlinkInfo and accesses
.version, but it never reads .state. As a result, the wrapper path behind
NvlinkInfo.state has no direct coverage even on systems where the test does
not skip.

Relevant paths:

• cuda_core/cuda/core/system/_nvlink.pxi:35
• cuda_core/tests/system/test_system_device.py:734

[mdboom]

Thanks for having your agent fight with my agent, @rwgk. ;)