feat(vector)!: add approx mode for RaBitQ search

[BubbleCal]

Feature

This PR adds a public approx_mode setting for vector search with fast, normal, and accurate values. The public API avoids exposing RaBitQ/HACC terminology while still allowing callers to choose the speed/accuracy tradeoff when the backing index supports it.

Implementation

• Adds ApproxMode to vector queries and threads it through Rust scanner, ANN proto serialization, Python query parsing, and FlatIndex distance calculator options.
• Implements RaBitQ behavior for each mode:
  • fast: force 1-bit query-time scoring for RaBitQ.
  • normal: preserve the existing search path.
  • accurate: use a 16-bit LUT accumulation path for the binary RaBitQ estimator.
• Extends query scratch with a wider accumulation buffer while keeping the existing QueryScratchCapacity::new(...) API compatible.
• Adds Python API support via approx_mode="fast" | "normal" | "accurate" and rejects invalid values.

Benchmark

IVF_RQ on dbpedia, num_bits=5, no refine_factor. The plot shows approx_mode=fast, normal, and accurate as separate curves.

Breaking Change

BREAKING CHANGE: The ANN protobuf schema now includes VectorApproxMode approx_mode on vector query serialization. Consumers that regenerate or explicitly match Lance's serialized ANN query proto should update to the new schema.

Validation

• cargo fmt --all
• cargo test -p lance-linalg simd::dist_table
• cargo test -p lance-index vector::bq::storage
• cargo test -p lance-index vector::storage::tests
• cargo test -p lance --features substrait test_query_roundtrip
• cargo test -p lance test_knn_approx_mode_defaults_and_setter
• cargo check --workspace --tests --benches
• cd python && make install
• cd python && uv run pytest python/tests/test_vector_index.py::test_vector_index_with_approx_mode python/tests/test_vector_index.py::test_vector_index_invalid_approx_mode
• cd python && uv run pytest python/tests/test_vector_index.py::test_create_ivf_rq_multi_bit_searches_l2_and_cosine
• git diff --check

[github-actions]

Important

This PR touches the Lance format specification.

Substantive changes to the format specification — the .proto definitions
and the spec docs under docs/src/format/ — require a PMC vote before merge.
Minor edits such as typo fixes, wording, or formatting are excluded; use your
judgment.

If this is a meaningful format change:

• Start a vote following the Lance community voting process.
  Format specification modifications need 3 binding +1 votes (excluding the
  proposer), held on GitHub Discussions, with a minimum voting period of 1 week.
• Once the vote passes, link the completed vote in this PR. It should not be
  merged until the vote is linked.

[codecov]

Codecov Report

❌ Patch coverage is 92.50000% with 54 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-linalg/src/simd/dist_table.rs	86.49%	37 Missing ⚠️
rust/lance-index/src/vector/bq/storage.rs	95.95%	12 Missing and 1 partial ⚠️
rust/lance/src/io/exec/ann_proto.rs	92.00%	2 Missing ⚠️
rust/lance/src/dataset/scanner.rs	95.65%	1 Missing ⚠️
rust/lance/src/index/vector/ivf/v2.rs	96.42%	1 Missing ⚠️

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