search: Phase 4 federation merge + ACL post-filter + 21 tests

fedIndex merges per-peer inverted indices (union posting lists per term) after
relabelling local DocIds to global gid = peer*1000 + local — dedupe by
(peer,doc-id) is automatic and positions survive, so ranking runs once over the
merge and interleaves peers by score. ACL is a post-rank filter over an injected
permit predicate (searchTfIdfAcl/topNTfIdfAcl/searchBm25Acl). Roadmap complete,
122/122.

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

lib/search/conformance.conf

@@ -23,6 +23,7 @@ PRELOADS=(
   lib/search/query.sx
   lib/search/parse.sx
   lib/search/rank.sx
+  lib/search/fed.sx
   lib/search/api.sx
   lib/search/testlib.sx
 )
@@ -32,4 +33,5 @@ SUITES=(
   "boolean:lib/search/tests/boolean.sx"
   "parse:lib/search/tests/parse.sx"
   "rank:lib/search/tests/rank.sx"
+  "integration:lib/search/tests/integration.sx"
 )