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/types.hpp

@@ -24,7 +24,20 @@ struct Frame {
     double   timestamp_sec{0.0};
     int64_t  frame_idx{-1};
     bool     eof{false};
-    bool     is_cut{false};   // true when a hard scene cut was detected before this frame
+    bool     is_cut{false};   // histogram: intra-scene camera-angle change (tracker reset)
+    bool     is_scene_boundary{false};  // TransNetV2: true shot/scene boundary (opt-in)
+    float    cut_score{0.f};  // histogram cut score = 1 - hist_corr (0=identical, ~1=cut); HUD/debug
+    float    bbox_upscale{1.f};  // multiply detector bboxes/landmarks by this to map back to
+                                 // original video resolution (>1 when dense_scale downscaled the frame)
+};
+
+// ── CutEvent ──────────────────────────────────────────────────────────────────
+// Emitted by SceneDetectorFunc when TransNetV2 localises a shot boundary, keyed
+// by the boundary frame's timestamp. eof=true is the shutdown sentinel.
+struct CutEvent {
+    double   timestamp_sec{0.0};
+    float    probability{0.f};   // sigmoid boundary score at the peak
+    bool     eof{false};
 };
 
 // ── ArcFace alignment ─────────────────────────────────────────────────────────
@@ -122,4 +135,13 @@ struct ActorGallery {
         std::vector<std::string> source_images;
     };
     std::vector<Actor> actors;
+
+    // Cached Platt-sigmoid calibration (see gallery/gallery_calibration.hpp),
+    // stored alongside the gallery in HDF5 so it never needs recomputing
+    // unless the reference embeddings actually change. calib_valid=false and
+    // calib_hash=0 means "not present in this file, compute it."
+    float    calib_a{10.f};
+    float    calib_b{-5.f};
+    bool     calib_valid{false};
+    uint64_t calib_hash{0};
 };