Fix bug due to rounding errors in periodicity computation

[efaulhaber]

On main, we first computed the periodic continuous coordinates and then converted to discrete cell coordinates. The first computation introduces rounding errors, sometimes resulting in coordinates outside the periodic box, which will then be converted to discrete cell coordinates outside the box as well. This leads to out-of-bounds errors.
This PR first computes the discrete cell coordinates and then applies a discrete modulo, so there are no rounding errors that can break anything.

[Copilot]

Pull request overview

This PR fixes a rounding error bug in periodicity computation by changing the approach: instead of first wrapping coordinates into the periodic box with periodic_coords and then computing cell indices (which can suffer from floating-point rounding at boundaries), it now computes cell indices from raw coordinates and then applies modular arithmetic to handle periodicity.

Changes:

• Unified cell_coords and periodic_cell_index into generic versions that no longer dispatch on cell list type, using mod(. - 2, n_cells) + 2 for periodic wrapping
• Removed FullGridCellList-specific specializations of cell_coords and periodic_cell_index from full_grid.jl
• Introduced nonperiodic_cell_coords as a separate step before periodic index wrapping

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/nhs_grid.jl	Refactored cell_coords to separate non-periodic coordinate computation from periodic wrapping; unified periodic_cell_index to use 2-based modulo for all cell list types
src/cell_lists/full_grid.jl	Removed FullGridCellList-specific cell_coords and periodic_cell_index specializations

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.40%. Comparing base (f25b0ab) to head (a55b471).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
- Coverage   86.06%   85.40%   -0.66%     
==========================================
  Files          15       15              
  Lines         739      733       -6     
==========================================
- Hits          636      626      -10     
- Misses        103      107       +4     

Flag	Coverage Δ
unit	85.40% <100.00%> (-0.66%)	⬇️

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[efaulhaber]

Codex Review

The PR appears intended to centralize periodic cell-index wrapping and make FullGridCellList use the same periodic path as other cell lists.

This is an AI-generated code review. Please verify the findings and summary before acting on them.
_{Review generated by codex-pr-review}