Port the SAR3D background gate, CLEAN and object boxes from Multi_depthnorm_clean

- BG_GATE_ENABLE: frame peak over its median background (sar3d_background); the
  GUI scales the frame with bg_gate_gain and its cloud threshold.
- SAR_CLEAN_ENABLE: CLEAN of a one-frame aperture on a 4x10x4 cm search grid over
  the whole scene; objects in their own scale (sar3d_objects).
- SAR_MARK_ENABLE: boxes by class around object groups (sar3d_marks), with the
  legend under the views.
- SAR_VOXEL_M as three steps X/Y/Z with the notebook defaults 0.025/0.02/0.02.
- GPU backend: image/download/load/project API, per-plane block reduction for the
  maximum, depth gate without FMA; sar3d host code built with -ffp-contract=off.
- References regenerated from notebook cells 1 and 5, with new CLEAN scenes and a
  synthetic box-rule case; docs updated.

data_acq_and_processing/processing/processors/include/sar3d_processor.hpp

@@ -8,11 +8,13 @@
 
 namespace radar::processing {
 
-// 3D SAR of the reference notebook Multi_antenna_demonstrate_depthnorm.ipynb on an NVIDIA
-// GPU. Publishes max-projections of the normalized volume as ImageF32 payloads:
-// sar3d_front (X-Y), sar3d_side (Z-Y), sar3d_top (X-Z) and sar3d_oblique (the
-// notebook's 3D scene view, axes in the plane of sar3d_oblique_basis), plus
-// sar3d_peak = [level, X, Y, Z] of the volume maximum.
+// 3D SAR of the reference notebook Multi_depthnorm_clean.ipynb on an NVIDIA GPU.
+// Publishes max-projections of the normalized volume, or of the CLEAN display, as
+// ImageF32 payloads: sar3d_front (X-Y), sar3d_side (Z-Y), sar3d_top (X-Z) and
+// sar3d_oblique (the notebook's 3D scene view, axes in the plane of
+// sar3d_oblique_basis); sar3d_peak = [level, X, Y, Z] of the shown peak; the frame's
+// sar3d_background ratio and the CLEAN sar3d_objects and sar3d_marks tables (see
+// python_app/gui/controllers/renderers/sar3d.py).
 class Sar3dProcessor final : public ProcessorInterface {
   public:
     Sar3dProcessor();