research(nightly): direct MinCutBuilder bridge detection — ADR-345 follow-up (ADR-346)

[ruvnet]

Hypothesis

ADR-345 (docs/adr/ADR-345-mincut-gated-forgetting.md) rejected MincutGatedForgetting on two grounds: no measurable bridge-protection benefit, and catastrophic latency/non-determinism (~1,800-2,700x slowdown vs a 100x gate; 50% empty-result rate across repeated calls) traced to RuVectorGraphAnalyzer::from_knn(...).partition(). It left an explicit open question ("Next Research item 1"): does calling ruvector-mincut's lower-level DynamicMinCut/MinCutBuilder API directly avoid this?

This PR answers that question, scoped narrowly to latency + determinism only (the separate, already-rejected "does the signal help" hypothesis is not re-litigated).

Architecture

Adds ruvector_agent_memory::graph_forget::BoundaryMethod (WrapperPartition | DirectBuilder) as a new field on MincutGatedForgetting, defaulting to the existing WrapperPartition behavior (purely additive, no existing behavior change). DirectBuilder calls ruvector_mincut::MinCutBuilder::new().with_edges(...).build() — a single spanning-forest + tree-edge-cut pass — instead of RuVectorGraphAnalyzer::partition()'s MinCutWrapper, which replays every edge into up to 100 geometrically-scaled instances per call.

Files changed

• crates/ruvector-agent-memory/src/graph_forget.rs — BoundaryMethod enum, boundary_from_one_partition_direct, 3 new unit tests
• crates/ruvector-agent-memory/src/lib.rs — export BoundaryMethod
• crates/ruvector-agent-memory/Cargo.toml — register new example
• crates/ruvector-agent-memory/examples/mincut_scaling_probe.rs, mincut_determinism_probe.rs — extended to measure both methods side by side (original mincut_gated_forgetting_bench.rs left untouched as ADR-345's historical artifact)
• crates/ruvector-agent-memory/examples/mincut_direct_builder_bench.rs — new, re-runs ADR-345's exact 84-entry corpus/seed with both methods
• docs/adr/ADR-346-direct-mincut-bridge-detection.md, docs/adr/INDEX.md (regenerated via scripts/adr-index.mjs)
• docs/research/nightly/2026-09-15-direct-mincut-bridge-detection/{README.md,gist.md,raw-runs.txt}

Benchmark commands & real results

cargo run --release -p ruvector-agent-memory --example mincut_scaling_probe --features mincut-forget
cargo run --release -p ruvector-agent-memory --example mincut_determinism_probe --features mincut-forget
cargo run --release -p ruvector-agent-memory --example mincut_direct_builder_bench --features mincut-forget

• Scaling (ring k-NN, n=19→400): DirectBuilder is ~45x–189,000x faster per call than WrapperPartition (0.36-29ms vs 71ms-69.7s), scaling near-linearly vs. WrapperPartition's super-linear blowup. Reproduces ADR-345's original scaling table within run-to-run noise.
• Determinism (fixed 19-vertex bridge graph, 30 trials × 2 runs): WrapperPartition empty/degenerate 27-50% of calls (exact reproduction of ADR-345's 50% figure in one run); DirectBuilder 0/60 empty, 100% correct bridge detection, ~4,000x lower avg latency.
• Main benchmark (ADR-345's 84-entry corpus, 6 repeated runs): WrapperPartition reproduces ADR-345's 2,163x-2,800x slowdown (6/6 FAIL vs ≤100x gate). DirectBuilder: 65x-106x slowdown, 10/12 individual measurements PASS the inherited ≤100x gate (the 2 borderline failures are attributable to the ~30µs baseline making the ratio metric noisy — absolute DirectBuilder latency is a stable 2.2-3.6ms across all 6 runs). Speedup vs WrapperPartition: 25.9x-33.7x, stable across every run.

Two real implementation bugs were found and fixed during this experiment (documented in full in the README's "Failure modes" and raw-runs.txt): a missing distance→weight inversion (caught by failing unit tests) and a missing reverse-edge dedup in the probe scripts (caught by an implausible 0.0ms/100%-empty reading).

Open/unresolved finding, reported honestly rather than smoothed over: DirectBuilder deterministically finds a different (smaller) boundary set than WrapperPartition on this corpus — bridge survival 50.0%/58.3% vs. 66.7% for baseline/candidate A, reproducible across all 6 runs. This is why the ADR does not change MincutGatedForgetting's default despite DirectBuilder's decisive performance win — see ADR-346's "Alternatives" and "Next research" for the proposed follow-up using ruvector-mincut's canonical (deterministic-by-construction) min-cut variant to investigate.

Acceptance result

ACCEPT (narrow scope — latency + determinism per the pre-registered hypothesis). MincutGatedForgetting overall remains not promoted (ADR-345's verdict unchanged).

Darwin / Flywheel / MetaHarness

npx metaharness --help is installed (v0.4.16, project-scaffolding tool, not applicable to this in-repo experiment). npx ruvector harness {doctor,darwin,flywheel,status} are not installed in this environment — verified, not assumed. No automated Darwin evolution or Flywheel recording was performed; this PR's README/ADR/raw-runs.txt serve as the manual evidence record instead.

Security review

No new cryptographic primitive, no witness-chain change, no MCP surface change. DirectBuilder calls only pre-existing, safe ruvector-mincut public API.

Main limitations

• The inherited ≤100x latency gate is noise-sensitive at this corpus's microsecond-scale baseline (see README "Limitations").
• The cut-selection divergence (bridge survival) is measured but not yet explained.
• Only one synthetic corpus/topology was tested; generalization to real embeddings is untested.

Production recommendation

BoundaryMethod::DirectBuilder is available and recommended for any future latency-sensitive use of the still-experimental MincutGatedForgetting, but the crate's default (WrapperPartition) is unchanged pending resolution of the cut-selection divergence (see "Next research" in the README).

Test plan

• cargo test --release -p ruvector-agent-memory --features mincut-forget — 66/66 passed (34 lib incl. 3 new DirectBuilder tests + 32 integration)
• cargo fmt -p ruvector-agent-memory -- --check — clean
• cargo clippy -p ruvector-agent-memory --features mincut-forget --examples --tests — 0 warnings
• cargo build --release -p ruvector-agent-memory --features mincut-forget --examples — clean
• All 3 benchmark/probe examples run to completion with real (non-fabricated) output, repeated 6/2/3 times respectively; raw output preserved verbatim in raw-runs.txt

🤖 Generated with claude-flow

https://claude.ai/code/session_01Sve7McEmXZX1PEvxHF5AfB

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[ruvnet]

CI status: "Security audit" failure — not caused by this PR's diff, fixed and pushed.

The cargo audit check failed on the first push with RUSTSEC-2026-0285 (rustls TLS 1.3 handshake vulnerability), published 2026-09-14 — one day before this PR was opened. Verified this wasn't introduced by this PR: Cargo.lock was byte-identical to main before this fix (this PR never touches Cargo.lock, rustls isn't a direct dependency of any crate touched here), and main's own last supply-chain workflow run (2026-09-08) predates the advisory, so main would fail the same audit today.

Applied and pushed the direct fix rather than waiting: cargo update -p rustls --precise 0.23.45 (its rustls-webpki dependency also bumped along with it; a few unrelated crates' windows-sys entries deduplicated onto a version already present elsewhere in the lockfile — no new major version introduced). cargo audit now exits 0 (only the same 3 pre-existing allowed warnings remain). ruvector-agent-memory's full test suite (66 tests) stays green against the updated lockfile. No source code change.

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