Added callbacks for node errors and fifo overflow

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

docs/static-network.md

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+# Static Networks
+
+`StaticNetwork` encodes the entire topology at compile time using a `make_network()` builder. Nodes and channel types are verified statically with zero runtime overhead.
+
+## Usage
+
+```cpp
+#include <kpn/kpn.hpp>
+using namespace kpn;
+
+static int  produce()        { return 42; }
+static int  double_it(int x) { return x * 2; }
+static void print_it(int x)  { std::cout << x << '\n'; }
+
+int main() {
+    auto src = make_node<produce>  ();
+    auto dbl = make_node<double_it>();
+    auto prn = make_node<print_it> ();
+
+    auto net = make_network(
+        edge(src, src.output<0>(), dbl, dbl.input<0>()),
+        edge(dbl, dbl.output<0>(), prn, prn.input<0>())
+    );
+
+    net.set_event_handler([](std::string_view name, NodeEvent ev, auto ts) {
+        // same API as Network
+    });
+
+    net.start();
+    std::this_thread::sleep_for(std::chrono::milliseconds(100));
+    net.stop();
+}
+```
+
+See `examples/10_static_hello_pipeline` and `examples/11_static_fanout`.
+
+## When to use
+
+| | `Network` | `StaticNetwork` |
+|---|---|---|
+| Topology known at | Runtime | Compile time |
+| Type checking | Runtime (`dynamic_cast`) | Compile time |
+| Overhead | Minimal | Zero |
+| Flexibility | Add nodes dynamically | Fixed at compile time |
+
+For most applications `Network` is sufficient. Use `StaticNetwork` when you need the absolute minimum overhead or want compile-time topology verification.