One-dimensional transient conduction through a layered wall with a receding hot face.
Backward Euler on a graded grid (fine at the hot face), layer interfaces on nodes, back face
adiabatic -- the conservative bound when the backing is not declared. The hot-face flux is a
function of the surface temperature, closed exactly each step: the interior is linear in the
surface temperature, so the surface balance is a scalar root.
Two recession modes:
* ablating (liner): the surface cannot exceed ``T_ablation``; once it gets there the flux
it cannot conduct away consumes material at ``H_surface`` J/kg (Landau problem);
* chemical (graphite): recession is the carbon mass flux the surface chemistry returns at
the solved surface temperature, not an energy balance.
A receding face is handled by remapping the profile onto the shortened wall each step.
A layer may carry ``cp_of(T)``: its heat capacity is then taken at each cell's temperature at the
start of every step (lagged one step, as backward Euler does with a property). Without it the
wall is exactly the constant-property model.
1.15.0