Add exploration queue for high selectivity multi-hop filter search

[ChenSunriseJiaBao]

Summary

• Add RejectAndNeedExpand variant to QueryVisitDecision enum to signal when exploration mode should be enabled
• Implement exploration queue mechanism in multi-hop search to maintain graph traversal through non-matching nodes when scratch.best is exhausted
• Add HighSelectivityBitmapFilter that uses RejectAndNeedExpand for high selectivity scenarios
• Add topk-high-selectivity-multihop-filter benchmark search type

Problem

When filter selectivity is very high (e.g., 99% of vectors filtered out), the standard multi-hop search can fail because:

1. scratch.best (priority queue of matching nodes) gets exhausted quickly
2. Search terminates with 0 or few results even though matching vectors exist

Solution

The exploration queue enables continued graph traversal through non-matching nodes when the primary queue is empty. This is critical for low match rate scenarios where we need to traverse through many non-matching nodes to find the few that match.

Test plan

• Run existing tests: cargo test --package diskann
• Run benchmark: cargo run --package diskann-benchmark -- run --input-file diskann-benchmark/example/async-multihop-high-selectivity-small.json --output-file output.json
• Verified 100% recall on high-selectivity test case

🤖 Generated with Claude Code

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