SDC2 evaluates time-dependent sources at the end of the step

Locations: Source/driver/Castro_advance_sdc.cpp:64,106,134-137 and
Source/problems/Castro_problem_source.cpp:19.

Finding

The driver computes node_time = time + dt_sdc[m] * dt, but the second-order
branch passes cur_time to do_old_sources at every node. cur_time is the
full timestep's endpoint. The problem source receives this time unchanged.

State-dependent sources may still use the correct staged state, but explicitly
time-dependent forcing is sampled at the wrong time. In particular, the cached
node-zero source is evaluated at the endpoint instead of the initial time. This
reduces temporal accuracy even on a single level. The fourth-order source branch
already passes node_time.

Reproducible numerical evidence

For a spatially uniform source S(t)=t, a step starting at zero should add
h*h/2. Both Lobatto node evaluations instead return h, so SDC2 adds h*h,
regardless of additional sweeps. Over a fixed interval this becomes a
first-order endpoint quadrature error.

This is an algebraic check of the source times, not a full manufactured-source
simulation.

Proposed fix

Pass node_time to the second-order source evaluation. Keep the time used to
retrieve staged state data distinct from the physical source time. Audit any
source that internally FillPatches before changing that argument; the true-SDC
diffusion contribution is already built separately as MOL fluxes.

Validation after fixing

Add a manufactured uniform time-dependent energy source to a copy of a suitable
existing problem. Check both Lobatto and Radau with SDC2, and verify exact
integration of a linear-in-time source to roundoff. Also test time-independent
sources and diffusion to catch ghost-fill regressions.

Start by tracing the SDC2 source calls at Source/driver/Castro_advance_sdc.cpp:64,106,134-137 and Source/problems/Castro_problem_source.cpp:19, keeping staged-state retrieval distinct from physical source time. Add the manufactured uniform time-dependent source to a suitable existing problem and validate Lobatto and Radau SDC2, including exact linear-in-time integration, time-independent sources, and diffusion without ghost-fill regressions.