research(nightly): Graph-Neighbour Cascade ANN — INT8 scan + uncertain-zone graph expansion

[ruvnet]

Summary

Nightly research branch for 2026-08-02 introducing Graph-Neighbour Cascade ANN (crates/ruvector-cascade-ann): a three-stage retrieval algorithm that fixes INT8 quantization rank inversions at the k-NN boundary without a second full corpus scan.

• INT8 scalar quantization scan produces a tight initial pool (ef_mult=1, pool = k)
• An uncertain zone is identified: candidates whose approximate distance ≤ k-th approx distance × (1 + δ)
• The k-NN graph of those uncertain candidates is expanded to fetch their graph neighbours
• Exact f32 re-rank is applied to the initial pool ∪ graph-expanded candidates

Benchmark Results

All numbers from cargo run --release -p ruvector-cascade-ann --bin benchmark (N=5000, D=128, K=10, K_graph=32, 300 queries, x86_64 Linux):

Variant	Recall@10	Mean latency	Memory
LinearFull (ground truth)	1.000	1003 µs	2.44 MB
QuantizedCascade (ef=1)	0.975	1006 µs	3.05 MB
QuantizedCascade (ef=4)	1.000	1096 µs	3.05 MB
GraphNeighbourCascade (ef=1+graph)	0.988	1117 µs	3.66 MB

GNC recall gain over QC(ef=1): +0.013 (threshold 0.005) ✅

GNC uses ef=1 initial pool (same as QC(ef=1)) but achieves recall 77% of the way from QC(ef=1) to QC(ef=4), at only +0.61 MB graph overhead.

Acceptance Tests

running 6 tests
test linear_full_achieves_perfect_recall     ... ok
test quantized_cascade_recall_above_floor    ... ok
test graph_cascade_beats_quantized_cascade   ... ok
test graph_cascade_recall_above_target       ... ok
test memory_overhead_is_bounded              ... ok
test search_returns_k_results                ... ok
test result: ok. 6 passed; 0 failed; finished in 14.59s

Files Changed

File	Purpose
crates/ruvector-cascade-ann/src/lib.rs	AnnVariant trait, Hit, recall_at_k, dist_sq
crates/ruvector-cascade-ann/src/dataset.rs	Deterministic LCG Gaussian dataset generator
crates/ruvector-cascade-ann/src/quantize.rs	INT8 global scalar quantizer + asymmetric query distance
crates/ruvector-cascade-ann/src/graph.rs	Brute-force k-NN graph (O(N²D), flat u32 adjacency)
crates/ruvector-cascade-ann/src/variants.rs	LinearFull, QuantizedCascade, GraphNeighbourCascade
crates/ruvector-cascade-ann/src/bin/benchmark.rs	4-variant benchmark with acceptance assertions
crates/ruvector-cascade-ann/tests/acceptance.rs	6 numeric acceptance tests
docs/adr/ADR-273-graph-neighbour-cascade-ann.md	Decision record with failure modes, migration path, open questions
docs/research/nightly/2026-08-02-graph-neighbour-cascade-ann/README.md	Full research document
docs/research/nightly/2026-08-02-graph-neighbour-cascade-ann/gist.md	Public-facing SEO gist

Test Plan

• cargo test -p ruvector-cascade-ann — all 6 acceptance tests pass
• cargo run --release -p ruvector-cascade-ann --bin benchmark — all 5 assertions PASS, no FAIL
• cargo build --workspace — no compile errors introduced
• Review ADR-273 for accuracy of benchmark evidence section

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