perf: use const reference in ConjugateGraph::Serialize loop to avoid unnecessary copies

Replace value copy with const reference for both the map entry and
the dereferenced UnorderedSet in the Serialize loop. This eliminates
N atomic shared_ptr increments and N full UnorderedSet copies when
serializing a conjugate graph with N nodes.

Signed-off-by: tianlan.lht <tianlan.lht@antgroup.com>
Co-authored-by: opencode <opencode@anthropic.com>
Signed-off-by: LHT129 <tianlan.lht@antgroup.com>

Author: LHT129

src/impl/conjugate_graph.cpp

@@ -155,8 +155,8 @@ tl::expected<void, Error>
 ConjugateGraph::Serialize(std::ostream& out_stream) const {
     out_stream.write((char*)&memory_usage_, sizeof(memory_usage_));
 
-    for (auto item : conjugate_graph_) {
-        auto neighbor_set = *item.second;
+    for (const auto& item : conjugate_graph_) {
+        const auto& neighbor_set = *item.second;
         uint64_t neighbor_set_size = neighbor_set.size();
 
         out_stream.write((char*)&item.first, sizeof(item.first));