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/conformance.conf

@@ -29,6 +29,7 @@ PRELOADS=(
   lib/search/fuzzy.sx
   lib/search/highlight.sx
   lib/search/stem.sx
+  lib/search/near.sx
   lib/search/api.sx
   lib/search/testlib.sx
 )
@@ -44,4 +45,5 @@ SUITES=(
   "fuzzy:lib/search/tests/fuzzy.sx"
   "highlight:lib/search/tests/highlight.sx"
   "stem:lib/search/tests/stem.sx"
+  "near:lib/search/tests/near.sx"
 )