add native 3D object detection using ZED SDK optimized models

Add engines::ZedNative class wrapping ZED SDK's TensorRT-optimized
object detection. On every frame grab, detected objects (label,
confidence, 3D position, 2D/3D bounding boxes) are serialized as a
flat float32 array and sent as a new "objects" dora output with
frame_id correlation to image_bgr.

Extract shared env-var helpers into src/util/env.h and expose
sl::Camera& via Camera::handle() for engine access.

Closes #3

src/zed/camera.h

@@ -37,6 +37,7 @@ public:
     Calibration calibration() const;
     sl::Mat& image();
     sl::Mat& point_cloud();
+    sl::Camera& handle();
 
 private:
     Config config_;