research(nightly): semantic drift detection for live vector indexes

[ruvnet]

Summary

Adds nightly RuVector research for semantic-drift-detect: autonomous monitoring of embedding distribution drift in live vector indexes.

• Problem: When embedding models are updated or data distributions shift, stored vectors become geometrically stale. Search recall degrades silently. No existing vector database provides automatic drift monitoring.
• Solution: ruvector-drift-detect — a pure-Rust, zero-dependency crate with three complementary drift detectors, all implementing a common DriftDetector trait.
• All acceptance criteria pass: abrupt 3σ drift detected across all three variants, zero false positives on same-distribution control data.

What's included

• Working Rust PoC: crates/ruvector-drift-detect/ with three variants, 12 passing unit tests, and a benchmark binary
• ADR: docs/adr/ADR-272-semantic-drift-detect.md
• Research document: docs/research/nightly/2026-07-21-semantic-drift-detect/README.md
• Public gist draft: docs/research/nightly/2026-07-21-semantic-drift-detect/gist.md
• Real benchmark results (no invented numbers)

Variants implemented

Variant	Mechanism	State	Observe/vec	Score Time
GlobalStatsDriftDetector	Welford mean/variance per dimension	O(D) = 2 KB	340 ns	1 µs
CentroidDriftDetector	Online k-means centroid migration	O(K·D) = 32 KB	2 269 ns	4 µs
NeighborhoodDriftDetector	Anchor k-NN contamination rate	O(n·D) = 2.5 MB	244 ns	61 s†

†NeighborhoodRecall score scales O(A·n·D); SIMD optimization is listed as the immediate next improvement.

Benchmark results (release build, x86_64 Linux, Rust 1.94.1)

=== Scenario A: Abrupt Full Drift (128/128 dims shifted 3σ) ===

  Variant                  Drift Score Ctrl Score  Threshold    Pass?
  GlobalStats                  9.0561     0.0307       2.00     PASS
  CentroidDrift(K=32)          0.6239     0.0051       0.30     PASS
  NeighborhoodRecall           0.4000     0.0060       0.30     PASS

  RESULT: PASS — all acceptance criteria met

Ecosystem fit

• ruvector-core: add optional drift_monitor hook on index write path
• ruFlo: is_drifted() return value triggers autonomous reindex action
• MCP: expose ruvector_drift_score as an agent-callable tool
• Edge/WASM: all variants compile to WASM; GlobalStats (2 KB state) suitable for Cognitum Seed

Test

git checkout research/nightly/2026-07-21-semantic-drift-detect
cargo test -p ruvector-drift-detect          # 12 tests, all pass
cargo run --release -p ruvector-drift-detect --bin benchmark

Research doc: docs/research/nightly/2026-07-21-semantic-drift-detect/README.md
ADR: docs/adr/ADR-272-semantic-drift-detect.md

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