Added callbacks for node errors and fifo overflow

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

docs/fanout.md

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+# Fan-out & Routing
+
+## FanoutNode
+
+Reads one item and pushes a copy to each of N output channels. All downstream nodes receive every item.
+
+```cpp
+auto fan = make_fanout<Image, 2>(/*capacity=*/8);
+
+net.connect("src",  src.output<0>(),  "fan",   fan.input<0>())
+   .connect("fan",  fan.output<0>(),  "nodeA", nodeA.input<0>())
+   .connect("fan",  fan.output<1>(),  "nodeB", nodeB.input<0>());
+```
+
+If one downstream channel overflows, that output drops the item independently — the other outputs are unaffected.
+
+See `examples/11_static_fanout`.
+
+## RouterNode
+
+Reads one item and pushes it to exactly one of N outputs, chosen by a selector function:
+
+```cpp
+auto router = make_router<Frame, 3>(
+    [](const Frame& f) -> std::size_t { return f.stream_id % 3; });
+
+net.connect("src",    src.output<0>(),     "router", router.input<0>())
+   .connect("router", router.output<0>(),  "nodeA",  nodeA.input<0>())
+   .connect("router", router.output<1>(),  "nodeB",  nodeB.input<0>())
+   .connect("router", router.output<2>(),  "nodeC",  nodeC.input<0>());
+```
+
+If the selector returns `>= N` the item is silently dropped.
+
+## FilterNode
+
+Reads one item and passes it downstream only when a predicate returns `true`:
+
+```cpp
+auto filt = make_filter<Frame>([](const Frame& f) { return f.valid; });
+
+net.connect("src",  src.output<0>(),  "filt", filt.input<0>())
+   .connect("filt", filt.output<0>(), "dst",  dst.input<0>());
+```