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_gui_profile_codec.py

@@ -82,7 +82,9 @@ class GuiProfileCodecTest(unittest.TestCase):
             ignore_socket_speed_enabled=True,
             look_angle_deg=6.5,
             frame_radius=3,
-            voxel_m=0.035,
+            voxel_x_m=0.035,
+            voxel_y_m=0.03,
+            voxel_z_m=0.025,
             bp_ground_depth_m=1.1,
             comp_range_power=0.5,
             use_compensation=False,