[codex] Add in-memory height above ground calculation

[iosefa]

Summary

• Add add_height_above_ground(existing_points, method=None, dtm=None) for in-memory HAG calculation.
• Support PDAL Delaunay HAG from classified ground points and DTM/raster HAG from a GeoTIFF.
• Add tests for both methods and validation paths.
• Document the required HeightAboveGround preprocessing workflow in the usage guide.

Why

Forest structure metrics require HeightAboveGround. This lets users add it after points are already loaded in memory instead of requiring all workflows to calculate HAG during read_lidar.

Impact

Users can normalize existing point arrays using classified ground points or a DTM raster before calculating canopy metrics.

Validation

• /Users/iosefa/miniforge3/envs/pyforestscan-qgis/bin/python -m pytest tests/test_filters.py -q
• /Users/iosefa/miniforge3/envs/pyforestscan-qgis/bin/python -m pytest -q

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 88.88889% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pyforestscan/filters.py	75.43%	14 Missing ⚠️
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Files with missing lines	Coverage Δ
tests/test_filters.py	99.10% <100.00%> (+1.43%)	⬆️
pyforestscan/filters.py	53.09% <75.43%> (+21.51%)	⬆️

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