v1.2.1

v1.2.1: Adaptive Mesh Coarsening for All Solver Types

- **Polar Coordinates Support**: Coarsening now works with polar coordinate systems
  - Both r and θ directions are coarsened with automatic aspect ratio optimization
  - Non-uniform FDM stencil for accurate discretization
  - Bilinear interpolation for inactive cells

- **Nonlinear Solver Support**: All nonlinear solvers now support coarsening
  - Picard iteration
  - Anderson acceleration
  - Newton-Krylov (Jacobian-free)

```yaml
materials:
  air:
    coarsen: true
    coarsen_ratio: 4  # Area reduction ratio (auto-calculates skip ratios)
```

- Significant speedup for large meshes with uniform regions
- Automatic skip ratio calculation maintains mesh aspect ratio

- Fixed coarsening mask generation for polar grids
- Fixed interpolation weight calculation for periodic boundaries