SDC2 conducts heat when diffuse_temp is disabled

[zingale]

Location: Source/hydro/Castro_mol_hydro.cpp:547-559.

Finding

The second-order MOL branch calls fill_temp_cond and mol_diffusive_flux
whenever DIFFUSION is compiled in. Unlike both fourth-order diffusion calls, it
does not test the runtime flag castro.diffuse_temp.

A diffusion-capable executable therefore conducts heat with diffuse_temp=0
when using SDC2. The runtime-disabled case can also lose its diffusion timestep
restriction while retaining the explicit diffusive update.

Evidence

The call is guarded only by #ifdef DIFFUSION.
Source/diffusion/diffusion_util.cpp:13-63 computes conductivity without
checking diffuse_temp, and Source/hydro/Castro_mol.cpp:317-350 adds the
resulting energy flux without checking it either. The fourth-order branch
explicitly tests diffuse_temp == 1.

A fresh 1-D debug executable also reproduced the ignored switch. With hydro
enabled to avoid report 02 and a fixed dt=1e-8, diffuse_temp=0 and
diffuse_temp=1 gave identical final printed diagnostics. Maximum temperature was
1.940592276 in both runs. Keeping diffuse_temp=0 but setting
diffuse_cond_scale_fac=0 changed that maximum to 1.940595995. README.md records
the inputs and commands.

Proposed fix

Wrap conductivity evaluation and mol_diffusive_flux in if (diffuse_temp).
Keep this independent of do_hydro, so diffusion-only runs remain supported.
The out-of-bounds zeroing in report 02 must also be fixed for that case.

Validation after fixing

Use the diffusion problem with a nonuniform temperature and zero velocity. With
hydro disabled, diffuse_temp=0 must leave energy unchanged, while
diffuse_temp=1 must diffuse. Test SDC2 and SDC4 using the same executable and
a small fixed timestep.