test(vector): assert PQ quantizer is honored on-disk through an Engine commit

[mosuka]

Summary

Adds the missing behavioral test that Product Quantization (PQ) is honored end-to-end through a VectorStore/Engine commit, asserting against a deterministic on-disk observable rather than only that search succeeds.

Follow-up of #790: that change propagated HnswOption::quantizer into HnswIndexConfig via from_hnsw_option, but the only coverage was converter-level unit tests. The existing pq_* tests build HnswIndexConfig directly and so bypass the from_hnsw_option path. A regression dropping quantizer from that converter (while keeping rerank_storage) would have gone unnoticed by any on-disk/behavioral test.

What the test does

test_pq_quantizer_honored_through_engine_commit (in laurus/tests/vector_segment_test.rs):

1. Builds a VectorIndexConfig whose HNSW field carries quantizer: QuantizationMethod::ProductQuantization { subvector_count: 2 } (dim = 4, so dim % subvector_count == 0).
2. Commits a dense, well-separated 16-document corpus (two clusters of 8, mirroring the stable test_hnsw_pq_search_returns_corpus_neighbour setup) through VectorStore — exercising from_hnsw_option.
3. Locates the committed .hnsw segment via Storage::list_files(), skips the 20-byte HNSW preamble (num_vectors:u64 + dim/m/ef:u32×3; asserts num_vectors == 16), and reads the LVS1 VectorSegmentHeader back.
4. Asserts QuantHeader::ProductQuantization with params.m == subvector_count and params.sub_dim == dim / subvector_count.

Why it is a real regression guard

The default QuantizationMethod is Scalar8Bit (quant_kind = 1). If from_hnsw_option dropped quantizer, the segment header would report quant_kind = 1 and the match arm would panic!. So the test fails iff PQ is not honored through the commit.

Scope

• Test-only. No production code, on-disk format, or public API change.
• No docs change (no behavior/feature change).

Verification

• cargo test -p laurus --test vector_segment_test — 2 passed (1 new)
• cargo clippy -p laurus --tests -- -D warnings — 0 warnings
• cargo fmt --check -p laurus — clean

Closes #798

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