Add create_vector_index for IVF_PQ, IVF_HNSW, IVF_SQ vector indices

[beinan]

Summary

• Add create_vector_index() public API that wraps lance.LanceDataset.create_index() with daft-lance's standard dataset construction pattern (URI, io_config, storage_options, version, etc.)
• Supports IVF_PQ, IVF_HNSW_PQ, IVF_HNSW_SQ, IVF_FLAT, IVF_SQ index types
• Configurable distance metric (L2, cosine, dot), num_partitions, num_sub_vectors, GPU accelerator
• Validates column exists and is a vector (fixed-size list) type before building
• Forwards **kwargs for advanced lance params (ivf_centroids, pq_codebook, target_partition_size, etc.)

Test plan

• 12 new tests covering: IVF_PQ creation, default type, cosine metric, custom name, replace=True/False, invalid column/type errors, vector search after index, case-insensitive type, kwargs forwarding
• All 290 existing tests pass (5 skipped)
• Pre-commit hooks pass

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