Add nonuniform grid support for interface boundary conditions with UpwindScheme

[utkuyilmaz1903]

Summary

This PR enables the default UpwindScheme() on nonuniform grids when interface or periodic boundary conditions are present. Previously, discretization failed with @assert length(bs) == 0 in the nonuniform upwind path.

Where the upwind stencil crosses an interface or a periodic wrap, stencil weights are now computed from the physical grid coordinates via calculate_weights, with cross-domain taps resolved through RefCartesianIndex and the coupled variable's grid. The uniform-grid path (DX <: Number) is unchanged.

Validation

check_boundarymap validates nonuniform interface setups through the 3-arg PDEBase hook used by discretize; a 2-arg method is kept for backward compatibility.

discretize throws ArgumentError for:

• UpwindScheme(order > 1) when vector grids are used with interface or periodic BCs
• misaligned interface coordinates (including the cross-domain ring case covered in tests)
• mixing a scalar step size with a vector grid at an interface
• mismatched uniform step sizes across connected domains (existing behavior)

Tests

New suite: test/pde_systems/MOL_1D_Interface_Upwind_NonUniform.jl
CI group: [Convection_Interface_NU] in test/test_groups.toml

Covers periodic and cross-domain interfaces, clustered grids, both wind directions, multi-domain chains, coordinate-mismatch guards, and related cases (108 tests).

[utkuyilmaz1903]

@xtalax @ChrisRackauckas As discussed in #562, here is the full implementation and comprehensive test suite for non-uniform interface and periodic boundary support with UpwindScheme.

[utkuyilmaz1903]

I will take a closer look this.

[utkuyilmaz1903]

@xtalax Just a gentle ping on this PR. This PR is ready.