feat: persistent-pipeline reuse — push_blocking, node introspection, stateful wrapper

Adds three pieces needed to build one KPN network and reuse it across many
replays/configs instead of tearing down and rebuilding per run:

- Channel<T>::push_blocking (+ IVariantChannel/VariantChannel forwarding):
  lossless backpressure push that waits for space instead of dropping when
  the ring is full. PyNode's run_loop now uses it so a downstream consumer
  lagging behind never silently drops a frame.
- PyNetwork::node_ptr / node_stats: raw node handle by name (for a binding
  to dynamic_cast to a concrete wrapper and call functor-specific runtime
  setters) and a per-node timing snapshot for profiling.
- ObjectVariantNodeWrapper: variant-node adapter for functors that need
  runtime-constructed state (a Config, a loaded gallery), mirroring
  VariantNodeWrapper's channel plumbing but backed by ObjectNode<Obj>.

Built and used downstream in scene-actor-extraction's sae_kpn Python replay
bindings for repeated threshold-sweep evaluation of the same pipeline.

include/kpn/python/object_variant_node.hpp

@@ -0,0 +1,149 @@
+#pragma once
+// ObjectVariantNodeWrapper — variant-node adapter for *stateful* functors.
+//
+// VariantNodeWrapper (variant_node.hpp) wraps Node<Func,...>, where Func is a
+// default-constructible NTTP callable. That doesn't fit nodes whose functor must
+// be constructed with runtime state (a Config, a loaded gallery, etc.) — those use
+// ObjectNode<Obj>, which takes `Obj& obj` at construction.
+//
+// This wrapper owns an Obj instance and exposes the same IVariantNode surface so a
+// stateful C++ node can live inside a PyNetwork. Build one via a factory that
+// constructs the functor from Python-supplied config, e.g.:
+//
+//   auto n = std::make_shared<ObjectVariantNodeWrapper<
+//                IdentityMatcherFunc, Variant, in<"tracked">, out<"matched">>>(
+//                fifo_cap, gallery, cfg);            // Obj ctor args forwarded
+//   net.add("identity_matcher", n);
+//
+// The wrapper mirrors VariantNodeWrapper's channel plumbing exactly; only the
+// underlying node type (PoolObjectNode, holding Obj&) differs.
+
+#include "../channel.hpp"
+#include "../node.hpp"
+#include "../variant_node.hpp"
+
+#include <memory>
+#include <stdexcept>
+#include <string>
+#include <tuple>
+#include <typeindex>
+#include <utility>
+#include <vector>
+
+namespace kpn {
+
+template<typename Obj, typename Variant,
+         typename InputTag  = in<>,
+         typename OutputTag = out<>>
+class ObjectVariantNodeWrapper;
+
+template<typename Obj, typename Variant,
+         fixed_string... InNames, fixed_string... OutNames>
+class ObjectVariantNodeWrapper<Obj, Variant, in<InNames...>, out<OutNames...>>
+    : public IVariantNode<Variant>
+{
+    using NodeT = ObjectNode<Obj, in<InNames...>, out<OutNames...>>;
+
+public:
+    using args_tuple   = typename NodeT::args_tuple;
+    using return_tuple = typename NodeT::return_tuple;
+
+    static constexpr std::size_t n_in  = NodeT::input_count;
+    static constexpr std::size_t n_out = NodeT::output_count;
+
+    // Owns the functor; forwards remaining args to Obj's constructor.
+    template<typename... ObjArgs>
+    explicit ObjectVariantNodeWrapper(std::size_t fifo_capacity, ObjArgs&&... obj_args)
+        : obj_(std::forward<ObjArgs>(obj_args)...)
+        , node_(obj_, fifo_capacity)
+        , in_channels_(n_in)
+        , out_channels_(n_out)
+        , out_type_indices_(n_out, std::type_index(typeid(void)))
+    {
+        init_inputs(std::make_index_sequence<n_in>{}, fifo_capacity);
+        init_out_types(std::make_index_sequence<n_out>{});
+    }
+
+    // Access the owned functor so callers can invoke its runtime setters (e.g. to
+    // change a threshold on a persistent pipeline without rebuilding the node).
+    Obj& functor() { return obj_; }
+
+    // ── INode ─────────────────────────────────────────────────────────────────
+    void start() override { node_.start(); }
+    void stop()  override { node_.stop(); }
+    bool running() const override { return node_.running(); }
+    const NodeStats& stats() const override { return node_.stats(); }
+    void set_name(std::string name) override { node_.set_name(std::move(name)); }
+    NodeSnapshot node_snapshot(const std::string& name, double elapsed_s) const override {
+        return node_.node_snapshot(name, elapsed_s);
+    }
+
+    // ── IVariantNode ──────────────────────────────────────────────────────────
+    std::size_t input_count()  const override { return n_in; }
+    std::size_t output_count() const override { return n_out; }
+
+    std::type_index input_type(std::size_t i) const override {
+        return in_channels_[i]->type_index();
+    }
+    std::type_index output_type(std::size_t i) const override {
+        return out_type_indices_[i];
+    }
+
+    std::shared_ptr<IVariantChannel<Variant>> input_channel(std::size_t i) override {
+        return in_channels_[i];
+    }
+
+    void set_output_channel(std::size_t i,
+                            std::shared_ptr<IVariantChannel<Variant>> ch) override {
+        set_output_impl(i, std::move(ch), std::make_index_sequence<n_out>{});
+    }
+
+private:
+    template<std::size_t... Is>
+    void init_inputs(std::index_sequence<Is...>, std::size_t cap) {
+        ((init_one_input<Is>(cap)), ...);
+    }
+
+    template<std::size_t I>
+    void init_one_input(std::size_t cap) {
+        using T = std::tuple_element_t<I, args_tuple>;
+        auto shared_ch = std::make_shared<Channel<T>>(cap);
+        node_.template set_input_channel<I>(shared_ch);
+        in_channels_[I] = std::make_shared<VariantChannel<T, Variant>>(std::move(shared_ch));
+    }
+
+    template<std::size_t... Is>
+    void init_out_types(std::index_sequence<Is...>) {
+        ((out_type_indices_[Is] =
+              std::type_index(typeid(std::tuple_element_t<Is, return_tuple>))), ...);
+    }
+
+    template<std::size_t... Is>
+    void set_output_impl(std::size_t port,
+                         std::shared_ptr<IVariantChannel<Variant>> ch,
+                         std::index_sequence<Is...>) {
+        bool matched = false;
+        ((Is == port && (set_output_at<Is>(std::move(ch)), matched = true)), ...);
+        if (!matched)
+            throw std::out_of_range("set_output_channel: port index out of range");
+    }
+
+    template<std::size_t I>
+    void set_output_at(std::shared_ptr<IVariantChannel<Variant>> ch) {
+        using T = std::tuple_element_t<I, return_tuple>;
+        auto* typed = dynamic_cast<VariantChannel<T, Variant>*>(ch.get());
+        if (!typed)
+            throw std::runtime_error(
+                "set_output_channel: type mismatch at output port " + std::to_string(I));
+        node_.template set_output_channel<I>(typed->raw_ptr());
+        out_channels_[I] = std::move(ch);
+    }
+
+    Obj   obj_;    // owned; node_ holds Obj& — declaration order keeps obj_ alive first
+    NodeT node_;
+    std::vector<std::shared_ptr<IVariantChannel<Variant>>> in_channels_;
+    std::vector<std::shared_ptr<IVariantChannel<Variant>>> out_channels_;
+    std::vector<std::type_index>                           out_type_indices_;
+};
+
+} // namespace kpn