Weight ctis.regrid by the voxel volume to conserve the integral

[roytsmart]

Summary

Makes ctis.regrid integral-conserving (density-preserving) instead of sum-conserving.

Each input voxel is weighted by its volume — the wavelength bin width times the field-pixel area, from the new SpectralPositionalVectorArray.volume_cell (named-arrays 2.3) — before the two-stage conservative regridding, and the output is divided by the output voxel volume. So values_input is treated as a density (e.g. a spectral radiance), and a resampled uniform field stays uniform.

values = values_input * coordinates_input.volume_cell(axis)
values = <1D conservative along wavelength>
values = <2D conservative along the spatial axes>
values = values / coordinates_output.volume_cell(axis)

Notes

• Bumps the pin named-arrays~=2.2 → ~=2.3 for volume_cell.
• The test now checks that the volume-weighted integral is preserved (to within the tolerance of the perturbation applied by the 2D conservative step).

Tests

ctis/_regrid_test.py: refine and coarsen cases, checking shape, finiteness, and integral conservation. Verified against a clean/released regridding (conservation is exact there).

🤖 Generated with Claude Code

https://claude.ai/code/session_01CtdKmedevWkDab6BWupXqQ

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2eb14d0) to head (ecfdeb5).

Additional details and impacted files

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##              main       #22   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           23        23           
  Lines          697       704    +7     
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+ Hits           697       704    +7     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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