research(nightly): semantic-drift-detector — PSI + coherence drift detection for agent memory

[ruvnet]

Summary

Adds nightly RuVector research for semantic-drift-detector — a pure-Rust semantic drift detection library for agent memory vector collections.

• Implements three composable DriftDetector variants under a shared trait: CentroidDrift (L2 centroid shift), PsiDrift (Population Stability Index on cosine histograms), CoherenceDrift (PSI + intra-window coherence delta)
• Zero external library dependencies; rand only for test/benchmark data generation
• All 13 tests pass; benchmark ACCEPTANCE RESULT: PASS (9/9 variant–dataset combinations, TPR=1.00, FPR=0.00)

Benchmark evidence (cargo run --release -p ruvector-drift --bin benchmark)

Hardware: x86-64, Intel Celeron N4020, Linux 6.18, rustc 1.94.1

Variant	N	D	mean_us	p50_us	p95_us	TPR	FPR	Acceptance
CentroidDrift	5000	128	1046	982	1338	1.00	0.00	PASS
PsiDrift	5000	128	2601	2556	2931	1.00	0.00	PASS
CoherenceDrift	5000	128	5813	5782	6946	1.00	0.00	PASS

Deliverables

• Crate: crates/ruvector-drift/
• ADR: docs/adr/ADR-199-semantic-drift-detector.md
• Research doc: docs/research/nightly/2026-06-11-semantic-drift-detector/README.md
• SEO gist: docs/research/nightly/2026-06-11-semantic-drift-detector/gist.md

Ecosystem connections

This PoC connects: RuVector vector search · ruvector-mincut (graph-cut compaction) · ruvector-temporal-tensor (window storage) · ruvector-verified (proof-gated re-embedding) · ruFlo (observe→compact→reindex workflow) · mcp-gate (agent introspection tools)

What's not claimed

• TPR=1.00/FPR=0.00 is on large synthetic drift (mean shift 2.0). Real workloads need calibration.
• No comparison to Milvus/Qdrant/etc. — no public drift detection benchmark exists for vector databases.
• CoherenceDrift's O(N²) coherence is sub-sampled to 200 vectors; introduces ~5–10% estimation error at N=5K.

Next steps (not in this PR)

• Incremental streaming detector (O(D) per vector)
• ruvector-drift-wasm WASM build
• MCP tool surface in mcp-gate
• ruFlo trigger schema

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