[FXC-6100] Fix restart of mass flow rate boundary conditions

[yifan-flex]

Mass Flow Rate Boundary Condition Restart Bug

Symptom

When a converged steady-state case with mass flow rate inflow/outflow boundaries is forked (restarted), the solver diverges within the first few pseudo steps. Mass flow rates change drastically from their converged values and supersonic flow
appears at outflow boundaries.

Root Cause

Mass flow rate boundaries are implemented as PI controllers that iteratively adjust a pressure ratio to match a target mass flow rate. The controller's internal state (the converged pressure ratio) is correctly saved to and loaded from the restart
file.
However, on a forked restart the pseudo-step counter is reset to zero. The controller's output expression uses a pseudoStep > 0 guard that was designed to apply an initial default value (pressure ratio = 1.0) on the very first step of a fresh run.
On restart, this guard incorrectly overrides the loaded converged state with the default value, causing a sudden jump in the applied boundary pressure — for example from ~1.15 to 1.0 for total pressure ratio at an inflow.
This mismatch between the converged flow field and the suddenly-wrong boundary condition causes immediate instability and divergence.

Fix

Move the pseudoStep > 0 guard from the output expression to the update expression. The output always uses the current controller state directly. At pseudo step 0 the update becomes a no-op, preserving whatever value the state holds — the default
1.0 on a fresh start, or the converged value on a restart. This is correct in both cases since at step 0 no flow data has been computed yet to base an update on.

Note

^{Cursor Bugbot is generating a summary for commit 4e32ee5. Configure here.}

[yifan-flex]

@johnmoore4 With the fix the update law will not get called for a forked case at the first pseudo step, so it still doesn't behave the same as a case continuously running. We could also remove the pseudoStep==0 guard, but this will make a fresh start case have a total pressure ~1.002. Which do you think is better?