Added callbacks for node errors and fifo overflow

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

examples/09_opencv_cellshade/main.cpp

@@ -38,7 +38,7 @@ static cv::Mat make_gradient(int W, int H) {
 
 // ── Pipeline functions ────────────────────────────────────────────────────────
 
-// [snippet: capture_fn]
+// --8<-- [start:capture_fn]
 static std::tuple<cv::Mat, cv::Mat> capture() {
     constexpr int W = 640, H = 480;
     static cv::VideoCapture cap;
@@ -75,7 +75,7 @@ static std::tuple<cv::Mat, cv::Mat> capture() {
     }
     return {frame.clone(), frame.clone()};
 }
-// [/snippet: capture_fn]
+// --8<-- [end:capture_fn]
 
 static cv::Mat to_gray(cv::Mat bgr) {
     cv::Mat gray;
@@ -120,7 +120,7 @@ static std::tuple<cv::Mat, cv::Mat> composite(cv::Mat edge_mask, cv::Mat colour)
 // The constructor opens both windows on the main thread (Wayland requirement).
 // operator() is called by step() whenever both channels have a frame ready.
 
-// [snippet: display_node]
+// --8<-- [start:display_node]
 class DisplayNode : public kpn::MainThreadNode<DisplayNode,
                                                kpn::in<"composite", "edges">,
                                                cv::Mat, cv::Mat> {
@@ -150,14 +150,14 @@ private:
         catch (const cv::Exception&) { return false; }
     }
 };
-// [/snippet: display_node]
+// --8<-- [end:display_node]
 
 // ─────────────────────────────────────────────────────────────────────────────
 
 int main() {
     using namespace kpn;
 
-    // [snippet: opencv_network]
+    // --8<-- [start:opencv_network]
     auto src       = make_node<capture>  (out<"colour","grey">{},            8);
     auto gray_node = make_node<to_gray>  (in<"bgr">{},   out<"gray">{},      8);
     auto edge_node = make_node<edges_fn> (in<"gray">{},  out<"edges">{},     8);
@@ -182,7 +182,7 @@ int main() {
        .connect("comp",  comp.template output<"result">(),     "display", disp.template input<"composite">())
        .connect("comp",  comp.template output<"edges">(),      "display", disp.template input<"edges">())
        .build();
-    // [/snippet: opencv_network]
+    // --8<-- [end:opencv_network]
 
     net.set_watchdog_interval(std::chrono::milliseconds(5000));
 #ifdef KPN_WEB_DEBUG
@@ -192,7 +192,7 @@ int main() {
     std::cout << "Cell-shading pipeline running. Press 'q' to stop.\n";
     std::cout << "Web debug UI: http://localhost:9090\n";
 
-    // [snippet: main_thread_step]
+    // --8<-- [start:main_thread_step]
     net.start();
 
     // Main thread drives display — imshow/waitKey stay on the GUI thread.
@@ -201,6 +201,6 @@ int main() {
         cv::waitKey(8); // yield event loop when no frame ready
 
     net.stop();
-    // [/snippet: main_thread_step]
+    // --8<-- [end:main_thread_step]
     return 0;
 }