perf(vector/search): PQ rerank pipeline not exposed: PQ → SQ → f32 not possible

[mosuka]

Round-3 perf push sub-issue (tracked under umbrella #536).

[M] PQ rerank pipeline not exposed: PQ → SQ → f32 not possible

• Where: HNSW Stage-2 rerank (hnsw/searcher.rs:493-522) widens int8 → f32 only. No
  path lets PQ candidates be reranked first against int8 then f32.
• Current behavior: Either PQ-only (Stage 3) or SQ-only (Stage 1) + optional f32 rerank
  (Stage 2). No 3-stage chain.
• Reference precedent: LanceDB IVF_PQ → row-group rerank; FAISS IndexRefineFlat over
  IndexPQ; ScaNN multi-stage.
• Suggested direction: Generic RerankPipeline (see L-sized rerank item) lets a
  configurable stack [PqScorer, SqScorer, F32Scorer]. Each stage narrows by factor,
  emits to next.
• Risk / scope: Large; part of the rerank refactor.

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ID: VS-30 — see ~/.claude/tasks/laurus/20260523_perf_round3_audit/task_list.md for the full Round-3 issue list.