search: highlight + snippet generation + 12 tests

highlight marks query-matching (normalized) tokens with [..]; snippet extracts a
context window around the first match. 178/178.

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

lib/search/api.sx

@@ -6,7 +6,7 @@
 ;; rankTfIdf, rankBm25, topNTfIdf, topNBm25, fedIndex, aclFilter, searchTfIdfAcl,
 ;; topNTfIdfAcl, searchBm25Acl, prefixTerms, prefixDocs, prefixRankTfIdf,
 ;; paginate, pageTfIdf, pageBm25, resultCount, editDist, fuzzyTerms, fuzzyDocs,
-;; fuzzyRankTfIdf.
+;; fuzzyRankTfIdf, highlight, snippet.
 
 (define
   search/src
@@ -27,4 +27,6 @@
     "\n"
     search/page-src
     "\n"
-    search/fuzzy-src))
+    search/fuzzy-src
+    "\n"
+    search/highlight-src))