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/tests/native_regressions.cpp

@@ -210,9 +210,12 @@ void test_configuration(const std::string& seed_path, const Workspace& workspace
     }
     const auto live_path = workspace.path / "live.json";
     std::ofstream(live_path) << Json{{"bscan_gain", 2}, {"settings_revision", 11},
+        {"sar3d_bg_gate_enabled", false}, {"sar3d_clean_enabled", false},
         {"legacy_gpr_mode", false}, {"relative_permittivity", "obsolete"}};
     processing::ProcessingLiveConfigLoader loader(live_path.string(), config);
-    require(loader.refresh_if_needed().bscan_gain == 2, "valid live config did not load");
+    const auto loaded = loader.refresh_if_needed();
+    require(loaded.bscan_gain == 2 && !loaded.sar3d_bg_gate_enabled && !loaded.sar3d_clean_enabled,
+        "valid live config did not load");
     const auto revision = loader.revision();
     for (const auto& invalid : {Json{{"bscan_gain", 1e100}},
              Json{{"gpr_input_positions", {0.0000001}}}, Json{{"bscan_gain", true}},