add a depth output

Publish sl::MEASURE::DEPTH as float32 [H, W] millimetres on every frame that
carries image_bgr, with the intrinsics, frame_id and timestamps the other
outputs carry.

The map is copied out rather than published in place: sl::Mat rows can be
padded, so the buffer is not necessarily H * W floats. inf and nan are cleared
to 0 in the same pass, matching the reference driver — a hole is meaningful and
a consumer cannot back-project a non-finite value.

Consumers that need 3D can now back-project depth with the published
intrinsics instead of receiving a point cloud.

Also drops the stale objects entry from the dataflow outputs, missed when
detection was removed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7g2cBJ5e3r15ZbJ9WZn8c

src/zed/camera.cc

@@ -119,6 +119,7 @@ bool Camera::grab() {
         return false;
     }
     zed_.retrieveImage(image_mat_, sl::VIEW::LEFT);
+    zed_.retrieveMeasure(depth_mat_, sl::MEASURE::DEPTH);
     zed_.retrieveMeasure(point_cloud_mat_, sl::MEASURE::XYZRGBA);
     return true;
 }
@@ -149,6 +150,10 @@ sl::Mat& Camera::image() {
     return image_mat_;
 }
 
+sl::Mat& Camera::depth() {
+    return depth_mat_;
+}
+
 sl::Mat& Camera::point_cloud() {
     return point_cloud_mat_;
 }