Add thread-safety to Truthiness algorithm statistics tracking (#2179)

The Truthiness algorithm accumulates statistics in mutable member
variables from a const `process()` method, creating race conditions when
called concurrently from multiple threads.

## Changes

- **`m_event_count`**: Changed from `uint64_t` to
`std::atomic<uint64_t>` for lock-free atomic operations
- **`m_stats_mutex`**: Added mutex to protect `m_average_truthiness`
updates during the online average calculation
- **Accessors**: `getAverageTruthiness()` now locks, `getEventCount()`
uses atomic `.load()`

```cpp
// In process()
{
  std::lock_guard<std::mutex> lock(m_stats_mutex);
  m_event_count++;
  m_average_truthiness += (truthiness - m_average_truthiness) / m_event_count;
}
```

Pattern follows existing thread-safe implementations in the codebase
(e.g., `UniqueIDGenSvc`).

<!-- START COPILOT CODING AGENT TIPS -->
---

💡 You can make Copilot smarter by setting up custom instructions,
customizing its development environment and configuring Model Context
Protocol (MCP) servers. Learn more [Copilot coding agent
tips](https://gh.io/copilot-coding-agent-tips) in the docs.

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: wdconinc <[email protected]>

Author: Copilot

src/algorithms/reco/Truthiness.cc

@@ -136,8 +136,11 @@ void Truthiness::process(const Truthiness::Input& input,
 
   // Update statistics using online updating formula
   // avg_n = avg_(n-1) + (x_n - avg_(n-1)) / n
-  m_event_count++;
-  m_average_truthiness += (truthiness - m_average_truthiness) / m_event_count;
+  {
+    std::lock_guard<std::mutex> lock(m_stats_mutex);
+    m_event_count++;
+    m_average_truthiness += (truthiness - m_average_truthiness) / m_event_count;
+  }
 
   // Report final truthiness
   debug("Event truthiness: {:.6f} (from {} associations, {} unassociated MC, {} unassociated RC)",

src/algorithms/reco/Truthiness.h

@@ -7,6 +7,8 @@
 #include <edm4eic/MCRecoParticleAssociationCollection.h>
 #include <edm4eic/ReconstructedParticleCollection.h>
 #include <edm4hep/MCParticleCollection.h>
+#include <atomic>
+#include <mutex>
 #include <stdint.h>
 #include <string>
 #include <string_view>
@@ -35,7 +37,8 @@ class Truthiness : public TruthinessAlgorithm, public WithPodConfig<TruthinessCo
 
 private:
   mutable double m_average_truthiness{0.0};
-  mutable uint64_t m_event_count{0};
+  mutable std::atomic<uint64_t> m_event_count{0};
+  mutable std::mutex m_stats_mutex;
 
 public:
   Truthiness(std::string_view name)
@@ -55,8 +58,11 @@ class Truthiness : public TruthinessAlgorithm, public WithPodConfig<TruthinessCo
   void process(const Input&, const Output&) const final;
 
   // Accessors for statistics
-  double getAverageTruthiness() const { return m_average_truthiness; }
-  uint64_t getEventCount() const { return m_event_count; }
+  double getAverageTruthiness() const {
+    std::lock_guard<std::mutex> lock(m_stats_mutex);
+    return m_average_truthiness;
+  }
+  uint64_t getEventCount() const { return m_event_count.load(); }
 };
 
 } // namespace eicrecon