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/conformance.conf

@@ -22,6 +22,7 @@ PRELOADS=(
   lib/search/index.sx
   lib/search/query.sx
   lib/search/parse.sx
+  lib/search/rank.sx
   lib/search/api.sx
   lib/search/testlib.sx
 )
@@ -30,4 +31,5 @@ SUITES=(
   "index:lib/search/tests/index.sx"
   "boolean:lib/search/tests/boolean.sx"
   "parse:lib/search/tests/parse.sx"
+  "rank:lib/search/tests/rank.sx"
 )