sar3d: fixed default scene window X [-2,2], Y [-0.5,2], Z [0,8] m

The grid bounds are now explicit config (sar3d_x/y/z_min/max_m) instead of
being derived from the antenna span and the depth gate. The depth mask
still zeroes voxels outside [min_depth, max_depth], so the wider Z window
just shows the masked wedge. The bound fields are double: the parity tool
recomputes the notebook-derived bounds (notebook_grid_bounds) and passes
them through the live config, and a float roundtrip shifted the axes by
~1e-7 m, flipping depth-mask knife-edge voxels against the reference npz.
Parity on frames 0034-0036 unchanged (1.3e-6, axes bit-match).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

data_acq_and_processing/gpr_cpp/examples/standalone_bp.cpp

@@ -135,8 +135,11 @@ int main(int argc, char** argv) {
         frames.push_back(ApertureFrame{a.get(), offs[f], wts[f]});
     }
 
-    // полнообластная ground-сетка (как одиночный 3D в ноутбуке)
-    Grid grid = build_full_region_grid(geom, bp, sar, mot.pitch_deg);
+    // ноутбучные границы сцены (размах антенн ±2 м, полоса глубин) вместо
+    // фиксированного окна по умолчанию
+    notebook_grid_bounds(geom, bp, sar, mot.pitch_deg, 2.0,
+                         sar.x_min_m, sar.x_max_m, sar.z_min_m, sar.z_max_m);
+    Grid grid = build_full_region_grid(sar);
     printf("сетка %dx%dx%d, счёт объёма...\n", grid.NX(), grid.NY(), grid.NZ());
 
     VolumeRaw vol = engine.aperture_volume(frames, grid);