fix(vector/search): select per-searcher top-k by distance, not underflow-prone similarity (#933)

[mosuka]

Summary

Fixes the in-searcher sibling of #927's defect 2: per-segment searchers sorted and truncated their results by similarity descending, but distance_to_similarity's exp(-d) (Euclidean/Manhattan) underflows to 0.0f32 at long range — collapsing every distant candidate into a tie whose unstable-sort order decided top-k membership arbitrarily (DotProduct similarly saturates via clamp(0,1)). Discovered by #931's frozen-behavior oracle: a 20-doc single-segment grid corpus returned [0, 1, 2, 3, 10] where brute force says [0, 1, 2, 3, 4]. The fan-out layer was fixed by #927/#928; the drop here happens earlier, at the per-segment truncate, where the fan-out cannot recover it.

Changes

All five per-searcher sort sites (distance is already in the same tuple/struct at each) now order ascending by distance with a doc_id tiebreak, exactly matching the fan-out's #927 convention:

• hnsw/searcher.rs: graph-path final_results sort (the repro site) + linear-fallback candidate sort
• flat/searcher.rs: field-filtered + unfiltered scan sorts
• ivf/searcher.rs: probed-cluster candidate sort

min_similarity filtering and every reported similarity value are unchanged — only ordering/truncation stops flowing through the underflow-prone conversion.

Tests

• New searcher_distance_sort_test.rs: HNSW / Flat / IVF pinned against exact brute-force top-k on the underflow-triggering grid fixture (discovery case q = 0.2 + mid/far queries; IVF probes all clusters so the expectation is exact).
• RED-proved via reversible swap: restoring the similarity sort reproduces the original [0, 1, 2, 3, 10] corruption.

Verification

• cargo fmt --check and cargo clippy --all-targets -- -D warnings clean on stable and 1.97.0
• cargo test -p laurus --lib: 1258 passed; --tests: all 65 binaries ok — no order-dependent test affected (quantized and distance orders agree wherever similarity does not underflow)
• wasm32 cargo check -p laurus-wasm --target wasm32-unknown-unknown: clean

Closes #933. Same defect class as #927 defect 2 (fan-out layer, PR #928). Found by and unblocks #931 (#650 rerank-pipeline campaign PR-1).