feat: add cuVS accelerator support

[Xuanwo]

This adds a first Python-side accelerator="cuvs" path for IVF_PQ without changing the existing accelerator="cuda" torch behavior. The new path wires cuVS training outputs back into Lance's existing index build flow and also enables the same backend selection through IndicesBuilder.prepare_global_ivf_pq.

The scope is intentionally narrow: build-time training only, IVF_PQ only, and no Rust ABI or query-path changes yet. I also added focused Python tests for cuVS dispatch and the new validation rules.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 55.74713% with 77 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/index/vector/builder.rs	6.66%	42 Missing ⚠️
rust/lance-index/src/vector/v3/shuffler.rs	72.86%	23 Missing and 12 partials ⚠️

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