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/api.sx

@@ -4,7 +4,7 @@
 ;; interpreter. Public Haskell entry points: indexDoc, lookupTerm, deleteDoc,
 ;; docFreq, allTerms, tokens, positioned, evalQuery, parseQuery, searchQuery,
 ;; rankTfIdf, rankBm25, topNTfIdf, topNBm25, fedIndex, aclFilter, searchTfIdfAcl,
-;; topNTfIdfAcl, searchBm25Acl.
+;; topNTfIdfAcl, searchBm25Acl, prefixTerms, prefixDocs, prefixRankTfIdf.
 
 (define
   search/src
@@ -19,4 +19,6 @@
     "\n"
     search/rank-src
     "\n"
-    search/fed-src))
+    search/fed-src
+    "\n"
+    search/prefix-src))