feat(index): stop HNSW greedy descent before level 0

[u70b3]

Summary

Fixes #5208.

Per the HNSW paper (Algorithm 2), the greedy (ef=1) descent should cover only levels max_level-1 down to 1; level 0 must be searched solely by the ef-bounded beam search starting from the level-1 entry point. HNSW::run_search and HNSW::run_search_acorn were greedily descending through level 0 first, which collapsed the beam's entry point into a deep local minimum — extra distance computations and measurably lower recall at low ef.

The SIFT1M benchmark in the issue (3 independent graph builds, paired bootstrap intervals) shows stopping before level 0 is a repeatable Pareto improvement: +3.7% recall@10 at ef=16, +0.5% at ef=32, statistically equal recall at higher ef, and lower paired latency throughout the sweep. Note the online builder's query path (OnlineHnswBuilder::search) and both build-time insert paths already used the paper-correct range — only these two query paths were off.

Test

New structural regression test test_greedy_descent_stops_before_level_0 drives run_search / run_search_acorn over a hand-built 6-node, 2-level graph with a counting DistCalculator: the entry node is alone at level 1, so any greedy step at level 0 shows up as extra distance computations (6 expected, 16 pre-fix). I used a structural assertion rather than a recall threshold because on synthetic uniform data the recall delta is not sign-stable across graph sizes — the SIFT1M numbers in the issue are the recall evidence.

Verification

• cargo test -p lance-index --lib: 887 passed
• cargo test -p lance --lib -- vector index::: 724 passed
• cargo clippy --all --tests --benches -- -D warnings: clean
• cargo fmt --all: clean

[codecov]

Codecov Report

❌ Patch coverage is 96.51163% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/vector/hnsw/builder.rs	96.51%	3 Missing ⚠️

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