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.

python_app/tests/test_processor_history.py

@@ -382,7 +382,7 @@ class Sar3dRecoveryTest(_ProcessorTestCase):
         self._live.gpr_speed_m_s = 0.0
         self._live.gpr_start_freq_mhz = 1500.0
         self._live.gpr_stop_freq_mhz = 6000.0
-        self._live.sar3d_voxel_m = 0.4
+        self._live.sar3d_voxel_x_m = self._live.sar3d_voxel_y_m = self._live.sar3d_voxel_z_m = 0.4
         self._live.sar3d_frame_radius = 0
         self._live.tx_geometry = [{"output_pos": 0, "x_m": -0.3, "y_m": 0.0, "z_m": 0.0}]
         self._live.rx_geometry = [{"input_pos": 0, "x_m": 0.2, "y_m": 0.4, "z_m": 0.0}]