Added callbacks for node errors and fifo overflow

Add new doc system which should/might deploy to pages.

include/kpn/inode.hpp

@@ -1,5 +1,6 @@
 #pragma once
 #include "diagnostics.hpp"
+#include <chrono>
 #include <functional>
 #include <string>
 #include <string_view>
@@ -10,6 +11,13 @@ namespace kpn {
 // invocation and keep running, false to stop the node.
 using NodeErrorHandler = std::function<bool(std::string_view node_name, std::exception_ptr)>;
 
+// Lightweight timestamp-only callback fired on per-node events.
+// The node name is known at registration time so it is not included here.
+using NodeEventCallback = std::function<void(std::chrono::steady_clock::time_point)>;
+
+// Event types reported to the network-level aggregate callback.
+enum class NodeEvent { Overflow, Closed };
+
 // ── INode — type-erased interface for Network / watchdog ─────────────────────
 
 struct INode {
@@ -21,6 +29,11 @@ struct INode {
     virtual NodeSnapshot node_snapshot(const std::string& name, double elapsed_s) const = 0;
     virtual void set_name(std::string name) = 0;
 
+    // Network-injected callbacks (slot 1 of each node's callback array).
+    // Default no-ops; overridden by PoolNode, PoolObjectNode, InterruptNode.
+    virtual void set_network_overflow_callback(NodeEventCallback) {}
+    virtual void set_network_closed_callback(NodeEventCallback)   {}
+
     // halt(): alias for stop() — immediate, discards in-flight work.
     virtual void halt() { stop(); }