search: Phase 3 ranking TF-IDF + BM25 + top-N + 23 tests

rankTfIdf and rankBm25 (configurable k1/b) over the candidate set, float scores
with deterministic DocId tiebreak; topNTfIdf/topNBm25. df/idf derived from
posting-list length. Tests cover tf/idf behavior, a BM25-vs-TF-IDF flip from
length-norm + tf-saturation, the b-parameter effect, tiebreak stability. 101/101.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/search/scoreboard.json

@@ -1,12 +1,13 @@
 {
   "lang": "search",
-  "total_passed": 78,
+  "total_passed": 101,
   "total_failed": 0,
-  "total": 78,
+  "total": 101,
   "suites": [
     {"name":"index","passed":18,"failed":0,"total":18},
     {"name":"boolean","passed":28,"failed":0,"total":28},
-    {"name":"parse","passed":32,"failed":0,"total":32}
+    {"name":"parse","passed":32,"failed":0,"total":32},
+    {"name":"rank","passed":23,"failed":0,"total":23}
   ],
-  "generated": "2026-06-06T19:42:39+00:00"
+  "generated": "2026-06-06T19:56:08+00:00"
 }