Add a per node error handler possibility

include/kpn/node.hpp

@@ -9,6 +9,7 @@
 #include <atomic>
 #include <chrono>
 #include <cstddef>
+#include <functional>
 #include <iostream>
 #include <memory>
 #include <stdexcept>
@@ -18,6 +19,10 @@
 
 namespace kpn {
 
+// Called when a node's function throws. Return true to skip the failed
+// invocation and keep running, false to stop the node.
+using NodeErrorHandler = std::function<bool(std::string_view node_name, std::exception_ptr)>;
+
 // ── INode — type-erased interface for Network / watchdog ─────────────────────
 
 struct INode {
@@ -99,6 +104,8 @@ public:
 
     void set_name(std::string name) override { name_ = std::move(name); }
 
+    void set_error_handler(NodeErrorHandler h) { error_handler_ = std::move(h); }
+
     const NodeStats& stats() const override { return stats_; }
 
     NodeSnapshot node_snapshot(const std::string& name, double elapsed_s) const override {
@@ -230,8 +237,13 @@ private:
                 break;
             } catch (const ChannelOverflowError& e) {
                 std::cerr << "[kpn] overflow: " << e.what() << "\n";
+            } catch (...) {
+                if (error_handler_ && error_handler_(name_, std::current_exception()))
+                    continue;
+                break;
             }
         }
+        stop_flag_.store(true, std::memory_order_relaxed);
     }
 
     // Pop all inputs into a tuple of argument values
@@ -287,6 +299,7 @@ private:
     std::atomic<bool>   stop_flag_{false};
     std::jthread        thread_;
     NodeStats           stats_;
+    NodeErrorHandler    error_handler_;
 };
 
 // ── ObjectNode — wraps a callable object (functor / class with operator()) ────
@@ -357,6 +370,8 @@ public:
 
     void set_name(std::string name) override { name_ = std::move(name); }
 
+    void set_error_handler(NodeErrorHandler h) { error_handler_ = std::move(h); }
+
     const NodeStats& stats() const override { return stats_; }
 
     NodeSnapshot node_snapshot(const std::string& name, double elapsed_s) const override {
@@ -474,8 +489,13 @@ private:
                 break;
             } catch (const ChannelOverflowError& e) {
                 std::cerr << "[kpn] overflow: " << e.what() << "\n";
+            } catch (...) {
+                if (error_handler_ && error_handler_(name_, std::current_exception()))
+                    continue;
+                break;
             }
         }
+        stop_flag_.store(true, std::memory_order_relaxed);
     }
 
     template<std::size_t... Is>
@@ -523,6 +543,7 @@ private:
     std::atomic<bool>   stop_flag_{false};
     std::jthread        thread_;
     NodeStats           stats_;
+    NodeErrorHandler    error_handler_;
 };
 
 // ── make_node overloads for callable objects ──────────────────────────────────