search: proximity/NEAR search + 9 tests

nearDocs k t1 t2 returns docs where both terms occur within k positions
(unordered); candidates from the posting intersection, filtered on positional
postings. 205/205.

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

lib/search/api.sx

@@ -6,7 +6,8 @@
 ;; rankTfIdf, rankBm25, topNTfIdf, topNBm25, fedIndex, aclFilter, searchTfIdfAcl,
 ;; topNTfIdfAcl, searchBm25Acl, prefixTerms, prefixDocs, prefixRankTfIdf,
 ;; paginate, pageTfIdf, pageBm25, resultCount, editDist, fuzzyTerms, fuzzyDocs,
-;; fuzzyRankTfIdf, highlight, snippet, stem, stemText, stemTokens, indexStemmed.
+;; fuzzyRankTfIdf, highlight, snippet, stem, stemText, stemTokens, indexStemmed,
+;; nearDocs.
 
 (define
   search/src
@@ -31,4 +32,6 @@
     "\n"
     search/highlight-src
     "\n"
-    search/stem-src))
+    search/stem-src
+    "\n"
+    search/near-src))