Added callbacks for node errors and fifo overflow

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

docs/error-handling.md

@@ -0,0 +1,84 @@
+# Error Handling & Events
+
+KPN++ provides three complementary layers for observing and reacting to failures.
+
+---
+
+## 1. Per-node error handler
+
+Called when a node's function throws an unhandled exception. Return `true` to skip the failed invocation and keep running; `false` to stop the node.
+
+```cpp
+--8<-- "examples/15_node_error_handler/main.cpp:error_handler"
+```
+
+When a node stops (either from `false` return or no handler installed), it:
+
+1. Disables its **input** channels — upstream stops pushing into dead queues.
+2. Disables its **output** channels — downstream nodes receive `ChannelClosedError` on their next pop, propagating the shutdown naturally through the graph.
+
+---
+
+## 2. Per-node overflow callback
+
+Fired with a timestamp each time an output push is dropped because the channel is full. The node name is known at registration so it is not included — keeping the callback zero-overhead when unused.
+
+```cpp
+--8<-- "examples/16_event_callbacks/main.cpp:per_node_callback"
+```
+
+!!! note
+    The callback is purely informational — the node always continues after an overflow. To stop the node on overflow, call `node.stop()` from inside the callback.
+
+A matching `set_closed_callback()` fires (also with just a timestamp) when the node stops due to a closed upstream channel:
+
+```cpp
+node.set_closed_callback([](std::chrono::steady_clock::time_point ts) {
+    std::cerr << "node stopped at t=" << ts.time_since_epoch().count() << '\n';
+});
+```
+
+Each node holds two callback slots per event type — one user-set (registered above) and one injected by the network (see below). Both fire independently.
+
+---
+
+## 3. Network-level event handler
+
+One callback for the whole network. Receives the node name (captured in a closure by the network at `build()` / `start()`), a `NodeEvent`, and a timestamp:
+
+```cpp
+--8<-- "examples/16_event_callbacks/main.cpp:network_event_handler"
+```
+
+`NodeEvent` values:
+
+| Value | Meaning |
+|---|---|
+| `NodeEvent::Overflow` | An output push was dropped (channel full) |
+| `NodeEvent::Closed` | The node stopped (crash or upstream close cascade) |
+
+The network handler and any per-node callbacks are **independent** — both fire when set.
+
+---
+
+## Complete example
+
+`examples/16_event_callbacks/main.cpp` shows a fast producer overflowing a slow consumer, with both a per-node overflow callback and a network-level event handler active simultaneously.
+
+Node functions:
+
+```cpp
+--8<-- "examples/16_event_callbacks/main.cpp:node_fns"
+```
+
+Per-node overflow callback:
+
+```cpp
+--8<-- "examples/16_event_callbacks/main.cpp:per_node_callback"
+```
+
+Network-level event handler:
+
+```cpp
+--8<-- "examples/16_event_callbacks/main.cpp:network_event_handler"
+```