research: benchmark hybrid (lexical+dense+graph) recall vs current FTS

Viktor asked to enhance the memory system with 'semantics' — remember concepts
(not just tokens) linked in a graph — and to prove, by benchmarking against the
current system, that it actually improves recall. A multi-phase research workflow
(18 agents) did landscape research, an adversarially-reviewed integration design,
a stratified eval set over the real 5,452-memory corpus, and a head-to-head
prototype-vs-current benchmark.

Result: hybrid (lexical FTS + dense embeddings, RRF-fused) beats FTS on every
overall metric, driven by a robust paraphrase win (recall@10 +0.350). Recommend
adopting lexical+dense; the concept graph is DEFERRED.

Post-run adversarial review correction (applied to all docs before commit): the
prototype's fusion config structurally barred the graph leg from the ranked top-k,
so the 'graph contributes nothing' ablation was a math artifact, NOT an empirical
result — the graph is UNEVALUATED, not disproven (deferred on cost+uncertainty).
Multi-hop deltas are not statistically significant. Glossary in CONTEXT.md; framing
in ADR-0001-0003; findings in ADR-0004-0006 + docs/research/.

Privacy: the corpus/queries/qrels/results are the user's real memories and stay
gitignored (data/, cache/, results/, build_eval_set.py); only harness code,
aggregate numbers, and synthetic examples are committed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/adr/0005-rrf-default-cc-challenger.md

@@ -0,0 +1,40 @@
+# Weighted RRF as the fusion default; convex combination as a benchmark challenger
+
+The hybrid read path must fuse three retrieval signals on **incomparable scales** — unbounded
+`ts_rank` (BM25), bounded cosine, and (phase 2) an arbitrary graph-proximity score — where the graph
+list is **sparse and often empty**. We adopt **weighted Reciprocal Rank Fusion** as the default
+fusion function: `score(d) = Σ_s w_s/(60 + rank_s(d))`, default `w_lex = w_dense = 1.0`,
+`w_graph = 0.35`, with the existing **importance** value applied as a *post-fusion prior multiplier*
+(`final = fused × (0.7 + 0.3·importance)` for `sort_by="relevance"`) — importance is a prior, **not**
+a fourth fused list.
+
+RRF is the right default because it is **score-scale-free** (no BM25-vs-cosine calibration to
+maintain), treats a missing leg as a clean **0** contribution (no missing-modality bias), is
+near-parameter-free (`k=60` is demonstrably uncritical across [10,100]), and **collapses to today's
+exact lexical ordering** when the dense/graph legs are empty — which is the SQLite-only
+graceful-degrade path ([ADR-0002](0002-api-postgres-first-sqlite-stays-lexical.md)) running the *same*
+code.
+
+## Considered options
+
+- **Convex combination / TM2C2** (Bruch et al., min-max normalization) — the literature consistently
+  shows it **edges RRF on nDCG/recall** when scores are calibratable (Weaviate switched its default to it).
+  It is the **standing challenger**. ⚠️ **Correction:** an earlier draft claimed "the benchmark ran CC
+  against RRF and RRF was chosen on our eval set" — **no CC results were actually produced or persisted in
+  this run.** RRF was adopted on *principled* grounds (scale-free, treats a missing/empty leg as a clean 0,
+  collapses to today's exact lexical ordering for the SQLite-only degrade path), **not** a measured
+  head-to-head. Benchmarking CC vs RRF on our eval set is an open follow-up — do it before locking fusion,
+  and especially if the graph is ever adopted or score distributions shift.
+- **Cross-encoder stage-2 re-rank** (e.g. `bge-reranker-v2-m3` over the fused top ~20–30) — a
+  *separate*, independently-gated stage, not a fusion function. Deferred; ship only if it clears both
+  the quality bar and the hot-path p95 budget on the GPU node.
+
+## Consequences
+
+- Fusion is ~30 lines over three top-N queries; the lexical leg reuses the existing
+  `plainto_tsquery`/`ts_rank` query + OR-broaden fallback verbatim.
+- The exact-stratum nDCG/MRR dip (~0.018/0.025, recall unaffected) is the known RRF cost of blending
+  one perfect hit with near-ties; a small **exact-match rank bonus** is the tunable recovery and a
+  cheap follow-up.
+- `k=60` is borrowed from TREC ad-hoc IR; a quick re-sweep on the eval set is worthwhile but the
+  literature says it is insensitive.