Calculate rumple on the voxel grid

[iosefa]

Rumple now takes a point array and voxel resolution and returns a raster and extent on the same XY grid as canopy cover, PAI, and FHD. Each cell measures the canopy surface using its eight neighbors. Missing neighborhoods stay NoData.

Adds tiled GeoTIFF output through metric="rumple", updated docs, and a notebook example with the resulting map. The tests cover the area calculation, grid alignment, missing data, and matching values across tile boundaries.

Also updates the PDAL pin so CI keeps the installed 3.5.x bindings instead of downgrading to 3.4.5.

Validation: 129 tests passed, with one existing slow integration test skipped. GitHub Actions passes on Python 3.11, 3.12, and 3.13. The notebook ran end to end on the bundled LiDAR data and wrote a 1 m rumple GeoTIFF.

This overlaps #54, which uses a different calculation. This version builds the canopy surface from column maxima and uses the Jenness surface-area method.

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Files with missing lines	Coverage Δ
pyforestscan/calculate.py	75.08% <100.00%> (+20.13%)	⬆️
pyforestscan/process.py	63.70% <100.00%> (+12.94%)	⬆️
tests/test_calculate.py	100.00% <100.00%> (ø)
tests/test_process.py	94.63% <100.00%> (+3.80%)	⬆️

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