feat: add ActsTrajectoriesMerger (#1824)

### Briefly, what does this PR introduce?
This PR pulls the ActsTrajectoriesMerger and non-owning JOmniFactory
Output support out of #1744.

### What kind of change does this PR introduce?
- [ ] Bug fix (issue #__)
- [x] New feature (issue #1744)
- [ ] Documentation update
- [ ] Other: __

### Please check if this PR fulfills the following:
- [ ] Tests for the changes have been added
- [ ] Documentation has been added / updated
- [ ] Changes have been communicated to collaborators

### Does this PR introduce breaking changes? What changes might users
need to make to their code?
No.

### Does this PR change default behavior?
No.

Author: wdconinc

src/extensions/jana/JOmniFactory.h

@@ -171,7 +171,7 @@ class JOmniFactory : public JMultifactory {
       fac.SetData<T>(this->collection_names[0], this->m_data);
     }
 
-    void Reset() override {}
+    void Reset() override { m_data.clear(); }
   };
 
   template <typename PodioT> class PodioOutput : public OutputBase {

src/global/tracking/ActsTrajectoriesMerger_factory.h

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: LGPL-3.0-or-later
+// Copyright (C) 2025 Wouter Deconinck
+
+#pragma once
+
+#include <ActsExamples/EventData/Trajectories.hpp>
+#include <JANA/JEvent.h>
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "extensions/jana/JOmniFactory.h"
+
+namespace eicrecon {
+
+class ActsTrajectoriesMerger_factory : public JOmniFactory<ActsTrajectoriesMerger_factory> {
+private:
+  Input<ActsExamples::Trajectories> m_acts_trajectories1_input{this};
+  Input<ActsExamples::Trajectories> m_acts_trajectories2_input{this};
+  Output<ActsExamples::Trajectories> m_acts_trajectories_output{this};
+
+public:
+  void Configure() {}
+
+  void ChangeRun(int32_t /* run_number */) {}
+
+  void Process(int32_t /* run_number */, uint64_t /* event_number */) {
+    for (const auto& traj : m_acts_trajectories1_input()) {
+      ActsExamples::Trajectories::IndexedParameters trackParameters;
+      for (auto tip : traj->tips()) {
+        trackParameters.insert({tip, traj->trackParameters(tip)});
+      }
+      m_acts_trajectories_output().push_back(
+          new ActsExamples::Trajectories(traj->multiTrajectory(), traj->tips(), trackParameters));
+    }
+    for (const auto& traj : m_acts_trajectories2_input()) {
+      ActsExamples::Trajectories::IndexedParameters trackParameters;
+      for (auto tip : traj->tips()) {
+        trackParameters.insert({tip, traj->trackParameters(tip)});
+      }
+      m_acts_trajectories_output().push_back(
+          new ActsExamples::Trajectories(traj->multiTrajectory(), traj->tips(), trackParameters));
+    }
+  }
+};
+
+} // namespace eicrecon