perf: ANN optimizations - SwissHashSet, AVX2 distance, insert_fast

[cave-g-f]

Port hot-path optimizations from parent repo PR 6610806 (commit 844664001):

• swiss_hash_set.h: New SSE2/SIMD-optimized Swiss table hash set for uint32_t keys with O(occupied_slots) clear and ~87.5% load factor. Replaces tsl::robin_set for inserted_into_pool_rs.
• neighbor.h: Add NeighborMinHeap::insert_fast() with AVX2-accelerated linear scan over a contiguous float distance array (_dist_arr) for faster insertion-point finding.
• index.cpp: Refactor iterate_to_fixed_point() to interleave prefetch, distance computation, and immediate insert; remove separate dist_scratch allocation; mark nodes visited and process neighbors under the same shared lock (eliminates redundant lock/unlock).
• pq_l2_distance.cpp: AVX2 inner loop for chunk-wise PQ centroid distance computation (8-wide _mm256_fmadd_ps).
• scratch.h/.cpp: Switch _inserted_into_pool_rs type to SwissHashSet and skip clear() on empty containers.

• Does this PR have a descriptive title that could go in our release notes?
• Does this PR add any new dependencies?
• Does this PR modify any existing APIs?
• Is the change to the API backwards compatible?
• Should this result in any changes to our documentation, either updating existing docs or adding new ones?

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