matprop.c: Blended material properties

Source under test: matprop.c

Reference-pinned test: tests/test_matprop.py::test_viscosity_recovers_configured_end_members

Anchor: Analytical limit: away from the rheological transition the blended viscosity recovers the configured end-members (melt_log10visc = 2, solid_log10visc = 21).

Tolerance: Calibrated absolute tolerances on log10(viscosity) at strongly molten and strongly solid nodes; see the test comments.

Discrimination guards: The recovered end-members differ by more than 10 decades (swapped-phase guard); positivity of density, heat capacity, conductivity, and expansivity at every node; the configured 4.0 W/m/K conductivity is recovered exactly.

Material properties are blended across the melting interval from the per-phase EOS evaluations. The viscosity end-members, the constant conductivity, and positivity of all blended quantities are pinned on the part-solidified state of the frozen-reference run.