search: prefix/wildcard queries + 14 tests

prefixTerms matches indexed terms by prefix (allTerms + isPrefixOf); prefixDocs
unions their docs; prefixRankTfIdf ranks via the matched terms. 136/136.

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

lib/search/conformance.conf

@@ -24,6 +24,7 @@ PRELOADS=(
   lib/search/parse.sx
   lib/search/rank.sx
   lib/search/fed.sx
+  lib/search/prefix.sx
   lib/search/api.sx
   lib/search/testlib.sx
 )
@@ -34,4 +35,5 @@ SUITES=(
   "parse:lib/search/tests/parse.sx"
   "rank:lib/search/tests/rank.sx"
   "integration:lib/search/tests/integration.sx"
+  "prefix:lib/search/tests/prefix.sx"
 )