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/gallery/gallery_store.hpp

@@ -2,9 +2,22 @@
 #include "types.hpp"
 #include <string>
 
-// Load/save the actor gallery from/to a JSON file.
+// Load/save the actor gallery. HDF5 (.h5/.hdf5) is the only format written;
+// legacy gallery.json files are still readable for backward compatibility but
+// save_gallery always writes HDF5 regardless of the requested extension.
 //
-// JSON format:
+// HDF5 layout:
+//   /embeddings   float32 [N, 512]   all actors' refs concatenated, row-major
+//   /offset       int64   [A]        first row of actor a in /embeddings
+//   /count        int32   [A]        number of refs for actor a
+//   /imdb_id /tmdb_id /jellyfin_id /name : variable-length string [A]
+//   /source_images : variable-length string [N], parallel to /embeddings rows
+//   /calibration/a, /b : scalar float32 attrs — Platt-sigmoid P(match|sim) fit
+//   /calibration/valid : scalar int8 attr (0/1)
+//   /calibration/hash  : scalar uint64 attr — hash of the embeddings the fit
+//                        was computed from; a mismatch means "recompute"
+//
+// Legacy JSON format (read-only):
 // {
 //   "actors": [
 //     {