feat(engine): add Python replay bindings, gallery pose-expansion, scene detection, embedding dumps

New C++ sources:
- kpn_bindings.cpp (sae_kpn): assembles the real face_tracker/identity_matcher/
  scene_tracker nodes inside a Python-driven KPN network via nanobind, for
  offline threshold-sweep replay against dumped embeddings (scripts/optimizer/).
- track_gallery.hpp: per-film gallery expansion — promotes a confidently-
  identified track's novel-pose reference views into an in-memory annex so
  later frames/tracks of that actor at similar poses are recognised, without
  touching the baked gallery.
- dump_embeddings.cpp: standalone exe that runs detect→embed only (no gallery,
  no matching) and dumps per-frame face embeddings + metadata to HDF5, so a
  parameter sweep can replay the expensive half once and vary tracking/matching
  config freely downstream.
- scene_detector.hpp / scene_detector_node.hpp: TransNetV2-based shot-boundary
  detection, opt-in alongside the always-on histogram cut detector.
- camera_position_change_detector_node.hpp, embedding_dump_node.hpp: supporting
  nodes for the above.

src/nodes/camera_position_change_detector_node.hpp

@@ -0,0 +1,73 @@
+#pragma once
+#include "types.hpp"
+#include "config.hpp"
+
+#include <opencv2/imgproc.hpp>
+#include <algorithm>
+#include <iostream>
+#include <string_view>
+
+// ── CameraPositionChangeDetectorFunc ──────────────────────────────────────────
+// KPN node: flags intra-scene camera-angle changes (hard cuts) by comparing each
+// frame's grayscale histogram to the previous frame's. When the normalised
+// histogram correlation drops below cut_threshold, the frame is marked with
+// Frame::is_cut = true.
+//
+// This is the pipeline's cut *trigger*: it owns cut detection so a single node
+// decides when the camera position has changed, and every downstream stage reads
+// the decision off Frame::is_cut (which rides with the frame's timestamp_sec /
+// frame_idx). The face_tracker consumes it to re-associate tracks across the cut
+// rather than blindly resetting; the track_gallery consumes it to bound
+// promotion to a single physical viewpoint.
+//
+// This is the always-on histogram cut — separate from the opt-in TransNetV2
+// scene detector, which localises true shot boundaries as Frame::is_scene_boundary.
+//
+// The node is a pure pass-through: it forwards the Frame unchanged except for
+// is_cut, so it slots between frame_source and face_detector without altering the
+// downstream contract. eof frames are forwarded immediately without processing.
+
+struct CameraPositionChangeDetectorFunc {
+    static constexpr std::string_view label() { return "camera_position_change_detector"; }
+
+    explicit CameraPositionChangeDetectorFunc(const Config& cfg)
+        : cut_threshold_(cfg.cut_threshold)
+    {
+        std::cerr << "[camera_position_change_detector] cut_threshold="
+                  << cut_threshold_ << "\n";
+    }
+
+    Frame operator()(Frame f) {
+        if (f.eof) return f;
+
+        cv::Mat gray;
+        cv::cvtColor(f.image, gray, cv::COLOR_BGR2GRAY);
+
+        cv::Mat hist;
+        const int   bins    = 64;
+        const float range[] = {0.f, 256.f};
+        const float* ranges = range;
+        cv::calcHist(&gray, 1, nullptr, cv::Mat(), hist, 1, &bins, &ranges);
+        cv::normalize(hist, hist, 1.0, 0.0, cv::NORM_L1);
+
+        if (prev_hist_valid_) {
+            double corr = cv::compareHist(prev_hist_, hist, cv::HISTCMP_CORREL);
+            // Cut score in [0,1]: 0 = identical to previous frame, ~1 = fully
+            // different. Rides on the Frame for the preview HUD / debugging.
+            f.cut_score = static_cast<float>(std::clamp(1.0 - corr, 0.0, 1.0));
+            f.is_cut = (corr < cut_threshold_);
+            if (f.is_cut)
+                std::cerr << "[camera_position_change_detector] cut at t="
+                          << f.timestamp_sec << "s  hist_corr=" << corr << "\n";
+        }
+        prev_hist_       = hist;
+        prev_hist_valid_ = true;
+
+        return f;
+    }
+
+private:
+    float   cut_threshold_;
+    cv::Mat prev_hist_;
+    bool    prev_hist_valid_{false};
+};