Bug: Coarsening+Viscosity

[glesur]

fix the computation of velocity gradients in the viscosity module when coarsening is enabled.

[Copilot]

Pull request overview

Fixes viscous velocity-gradient computations when grid coarsening is enabled by applying coarsening-aware effective cell spacing before forming derivatives/stress terms.

Changes:

• Introduce per-cell effective dx1/dx2/dx3 in Viscosity::AddViscousFlux and scale them by the local coarsening factor.
• Use the corrected spacings consistently across IDIR/JDIR/KDIR sweeps for gradient, divergence, and stress-tensor terms.

Suppressed comments (1)

src/fluid/viscosity.cpp:464

• dx3 is computed unconditionally but only used when DIMENSIONS == 3 in this sweep. In 2D builds this can produce an unused-variable warning. Mark it [[maybe_unused]] to keep builds warning-clean.

      real dx1 = dx1Array(i);
      real dx2 = dx2Array(j);
      real dx3 = dx3Array(k);

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[svalat]

Looks ok to me. Ready to merge.