feat(engine): HDF5-native galleries with embedded calibration; TensorRT backends; scene detection

Gallery format switches from JSON to HDF5 exclusively (JSON read-only kept for
back-compat): save_gallery always writes HDF5, and the fitted Platt-sigmoid
calibration (a, b, valid, hash) is now embedded directly in the gallery file
instead of a sidecar .calib_cache.json — identity_matcher reads it from the
loaded gallery and writes back only when the embeddings actually changed
(hash mismatch), skipping the O(n^2) refit otherwise.

Also includes: TensorRT inference backend support (ort_backend.cpp,
trt_backend.cpp), gemm_backend improvements, TransNetV2-based scene-boundary
detection wired through frame_source/face_tracker/main, and CMake build
target updates for the new sources.

Bumps the KPN submodule to feature/persistent-pipeline-reuse (push_blocking
backpressure, node_ptr/node_stats introspection, ObjectVariantNodeWrapper for
stateful functors) — needed by the optimizer's sae_kpn Python bindings.

src/nodes/scene_tracker_node.hpp

@@ -26,6 +26,10 @@ struct SceneTrackerFunc {
         std::cerr << "[scene_tracker] extinction_sec=" << extinction_sec_ << "\n";
     }
 
+    // Runtime setter for pipeline reuse across a sweep. Also clears the active-actor
+    // state so a re-run starts clean (no carry-over from the previous config's film).
+    void set_extinction_sec(double s) { extinction_sec_ = s; active_.clear(); }
+
     SceneAnnotation operator()(MatchedSceneFrame mf) {
         if (mf.source.eof) return {0.0, {}, /*eof=*/true};