Support higher-order (P>1) B-spline PIC deposition on tripolar grids

[mochell]

Background

Issue #60 generalized particle→grid deposition to arbitrary B-spline order P=1,2,3 on the live MeshGrids/CartesianGrid path. The implementation guards against spline_order > 1 on tripolar grids: WaveGrowth2D errors early if a tripolar grid is combined with a higher-order spline, because the existing N_TripolarNorth seam handling in ParticleInCell.push_to_grid! only remaps a single deposition point across the fold.

Problem

A higher-order stencil deposits to (P+1)^2 points, and points straddling the tripolar seam each need independent remapping; the north-fold also flips the x-index, so stencil offsets do not map linearly across the seam. The current single-point TripolarNorthBoundary logic does not generalize.

Task

Generalize the tripolar seam remap so each of the (P+1)^2 stencil points is folded/remapped independently (handling the x-flip), making P=2 (TSC) and P=3 (cubic) deposition correct and conservative on tripolar grids. Then remove the constructor guard in src/Models/WaveGrowthModels2D.jl.

References

• src/ParticleInCell.jl — push_to_grid! (N_TripolarNorth branch), TripolarNorthBoundary
• src/Models/WaveGrowthModels2D.jl — the spline_order > 1 tripolar guard added in Generalize PIC deposition to arbitrary B-spline order #60
• Parent issues: Cartesian PIC propagation focuses wave energy onto the grid axes (breaks axisymmetry) #59 (anisotropy), Generalize PIC deposition to arbitrary B-spline order #60 (B-spline generalization)