search: synonym / query expansion + 9 tests

A synonym map [(Term,[Term])] expands a query term to itself + synonyms
(expandTerm); synDocs unions and synRankTfIdf ranks the expanded set. 214/214.

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

lib/search/api.sx

@@ -7,7 +7,7 @@
 ;; topNTfIdfAcl, searchBm25Acl, prefixTerms, prefixDocs, prefixRankTfIdf,
 ;; paginate, pageTfIdf, pageBm25, resultCount, editDist, fuzzyTerms, fuzzyDocs,
 ;; fuzzyRankTfIdf, highlight, snippet, stem, stemText, stemTokens, indexStemmed,
-;; nearDocs.
+;; nearDocs, expandTerm, synDocs, synRankTfIdf.
 
 (define
   search/src
@@ -34,4 +34,6 @@
     "\n"
     search/stem-src
     "\n"
-    search/near-src))
+    search/near-src
+    "\n"
+    search/syn-src))