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/conformance.conf

@@ -30,6 +30,7 @@ PRELOADS=(
   lib/search/highlight.sx
   lib/search/stem.sx
   lib/search/near.sx
+  lib/search/syn.sx
   lib/search/api.sx
   lib/search/testlib.sx
 )
@@ -46,4 +47,5 @@ SUITES=(
   "highlight:lib/search/tests/highlight.sx"
   "stem:lib/search/tests/stem.sx"
   "near:lib/search/tests/near.sx"
+  "syn:lib/search/tests/syn.sx"
 )