Add digging despike filters and bounded inpainting

[Idate96]

Summary

• add a near-base height filter and a two-stage positive spike filter for BASE-centered digging maps
• make chained plugin layers resolve dependencies correctly and reset cached layers when the base map changes
• bound inpainting to small holes on top of the despiked surface instead of filling large gaps away
• add synthetic regressions plus a DIG3D shovel snapshot test and dataset note

Testing

• colcon build --packages-select elevation_mapping_cupy --symlink-install
• direct Python execution of test_near_base_height_filter.py, test_positive_spike_filter.py, and test_positive_spike_filter_shovel_snapshot.py

Notes

• pytest collection for the despike test modules still hits the existing container-local cupy / importlib_metadata issue, so the regressions were executed directly under Python instead.