A material cannot both melt and shear-thin

What is wrong. The thermal plug-in writes a melting material's MPM viscosity (its melt law, from alloy/plugins/thermal/phase_materials.py) before each solver step. The Herschel-Bulkley update rewrites the viscosity of every particle with fluid.consistency inside the step. On a material with both, the melt law is lost without a warning.

The number. No library material has both today (dark_chocolate and porridge_oat melt, none of them has K and n); tests/solvers/mpm_rheology/test_herschel_bulkley.py::test_no_library_material_both_shear_thins_and_melts keeps it that way. A custom card with both would run with the HB viscosity only.

Why. Both features write the same per-particle array, model.mpm.viscosity, and there is no rule yet for combining a temperature factor with a shear-rate law.

Workaround. Give a melting material its melt law only (no consistency / flow_index), or a shear-thinning paste no melt law.

The fix and when. When the first material needs both (melted chocolate is shear-thinning): scale K by the thermal factor inside the Herschel-Bulkley kernel, or refuse the combination at build. Not scheduled.