Add reference-retriever spec and retrieval-determinism sections

v1.0.1-addendum.md

@@ -88,7 +88,41 @@ functional subset (1,429/1,743 test questions), and the interval × stale cell
 
 ---
 
-### 3. Retrieval determinism: two rules a reimplementation must match
+### 3. Reference retriever: full specification
+
+A reviewer noted that the BFS+BM25 reference retriever is underspecified on
+indexing and ranking. The paper has no room for it, so the complete spec is
+here. This describes the *reference baseline* whose outputs ship in
+`baselines/` — it is not part of the benchmark, and nothing in the benchmark
+depends on it.
+
+**Indexing.** One BM25 document per KG quadruple (550,376 documents). Document
+text is `label(s) label(r) label(o)`, lowercased and `\w+`-tokenised;
+unresolvable QIDs fall back to the raw QID string. **The validity window is
+never indexed** — time enters only through the graph filter, which is why a
+stale variant and its gold counterpart are lexically identical (§2).
+
+**BM25.** Okapi, `k1 = 1.5`, `b = 0.75`, with Lucene-style smoothed IDF
+`log(1 + (N - df + 0.5) / (df + 0.5))`, `N = 550,376`. Query terms are
+deduplicated (no query-term-frequency / `k3` component). `avgdl` is computed
+over the same corpus.
+
+**Candidate generation (BFS).** The anchor is the gold first-hop subject
+(oracle); the non-oracle variant uses a deliberately simple longest-substring
+linker over entity labels, 60.2% anchor recall. Expansion is undirected via the
+entity index, with a per-hop temporal filter `valid_at(t_query)` — exact-year
+equality, since the source KG is point-in-time. Per-hop caps are 60 / 200 / 400
+for hops 1 / 2 / 3, applied in CSV load order **before** BM25 re-ranking. The
+pool is deduplicated on `(s, r, o)`, keeping the first time-version.
+
+**Ranking.** BM25 re-ranks the surviving pool. Candidates are grouped by
+`(s, r, o)`, rendered as one evidence line each, sorted by score descending, and
+truncated to `k = 15` (`k = 25` for the 3-hop ablation). The stale-blind
+ablation is the identical architecture with the temporal filter off.
+
+---
+
+### 4. Retrieval determinism: two rules a reimplementation must match
 
 Neither of these changes any published number, but both are load-bearing if you
 rebuild the reference retriever and expect to reproduce our figures.
@@ -134,7 +168,7 @@ iv = [r for r in rows if r["operator_type"] == "interval"]
 print(len(iv), sum(str(r["t_query"]).startswith(str(r["answer"])) for r in iv))
 ```
 
-**The retrieval-side numbers (§2, §3, and the paper's tables) are *not*
+**The retrieval-side numbers (§2–§4, and the paper's tables) are *not*
 regenerable from this release.** They require the reference retriever and
 generator, which are not included here. They are instead shipped as *outputs*:
 every figure behind those claims is in `baselines/`, so each is checkable