search: result pagination (offset/limit) + 12 tests

paginate windows a ranked list (take lim . drop off); pageTfIdf/pageBm25 and
resultCount. 148/148.

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

lib/search/conformance.conf

@@ -25,6 +25,7 @@ PRELOADS=(
   lib/search/rank.sx
   lib/search/fed.sx
   lib/search/prefix.sx
+  lib/search/page.sx
   lib/search/api.sx
   lib/search/testlib.sx
 )
@@ -36,4 +37,5 @@ SUITES=(
   "rank:lib/search/tests/rank.sx"
   "integration:lib/search/tests/integration.sx"
   "prefix:lib/search/tests/prefix.sx"
+  "page:lib/search/tests/page.sx"
 )