Add a unified observability interface for applications with multiple networks

include/kpn/diagnostics.hpp

@@ -134,6 +134,15 @@ struct NodeSnapshot {
     double      cpu_util_pct;     // exec_ms / (exec_ms + blocked_ms) * 100
 };
 
+// ── Cross-network snapshot (used by DebugHub) ─────────────────────────────────
+
+struct NetworkSnapshot {
+    std::string                  name;
+    std::vector<NodeSnapshot>    nodes;
+    std::vector<ChannelSnapshot> channels;
+    double                       elapsed_s;
+};
+
 // ── Resource statistics + snapshot ───────────────────────────────────────────
 
 struct ResourceSnapshot {