Default search_docs to dense; make rerank opt-in (#17)

Co-authored-by: claude <[email protected]>

eval/results/post-prefix.md

@@ -0,0 +1,21 @@
+# Retrieval eval — k=5
+
+_22 hand-curated queries, generated 2026-09-30 02:40:07_
+
+Live container after nomic prefixes + heading-recursive chunking
+(`hvm-docs` image `b200c5f76c56`, Watchtower 2026-09-30T02:36Z).
+Compared with [`baseline.md`](baseline.md) (2026-05-22, pre-prefix).
+
+| Retriever | P@1 | MRR | Recall@5 | nDCG@5 | avg latency |
+| --- | ---: | ---: | ---: | ---: | ---: |
+| `dense` | 0.955 | 0.966 | 0.985 | 0.972 | 183ms |
+| `bm25` | 0.864 | 0.882 | 0.909 | 0.886 | 8ms |
+| `hybrid_rrf` | 0.955 | 0.961 | 0.955 | 0.955 | 113ms |
+| `bm25+rerank` | 0.773 | 0.827 | 0.848 | 0.816 | 162ms |
+| `hybrid_rrf+rerank` | 0.727 | 0.813 | 0.894 | 0.821 | 254ms |
+
+Dense flipped from worst (May MRR 0.539) to best. Rerank now hurts.
+`search_docs` therefore defaults to dense; rerank is opt-in via
+`RERANK_ENABLED`.
+
+Dense P@1 miss: `create a user account` (MRR 0.250).

eval/trace.py

@@ -86,7 +86,7 @@ def main() -> int:
         expected = [(e["bundle_id"], e["page_id"]) for e in q["expected"]]
         dense_pages = dense_r.retrieve(q["query"], k=50)
         bm25_pages = bm25_r.retrieve(q["query"], k=50)
-        ranked = bm25_pages or dense_pages  # HVM default retrieval is BM25-first
+        ranked = dense_pages or bm25_pages  # HVM default retrieval is dense-first
         top1 = ranked[0] if ranked else None
         p1 = p_at_1(ranked, expected)
         rows.append({