Add OpenSearch search backend with hybrid neural+BM25 support

Extract a SearchBackend trait from the existing RocksDB search code and
add an OpenSearch implementation supporting cross-room search, relevance
ranking, fuzzy matching, English stemming, and optional hybrid
neural+BM25 semantic search using sentence-transformers.

Fix macOS build by gating RLIMIT_NPROC and getrusage to supported
platforms.

.github/workflows/main.yml

@@ -332,6 +332,19 @@ jobs:
           {"sys_target": "x86_64-v4-linux-gnu", "feat_set": "default"},
         ]
 
+  opensearch-tests:
+    if: >
+      !failure() && !cancelled()
+      && fromJSON(needs.init.outputs.enable_test)
+      && !fromJSON(needs.init.outputs.is_release)
+      && !contains(needs.init.outputs.pipeline, '[ci no test]')
+
+    name: OpenSearch
+    needs: [init, lint]
+    uses: ./.github/workflows/opensearch-tests.yml
+    with:
+      checkout: ${{needs.init.outputs.checkout}}
+
   package:
     if: >
       !failure() && !cancelled()