Fix mid_step ignoring t_step_start in restart roundtrip

mid_step = t_step_stop // 2 produced an invalid Phase 1 run when
t_step_start > 0, since the midpoint could fall before the start step.
Use (t_step_start + t_step_stop) // 2 so Phase 1 always has a valid range.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sbryngelson

toolchain/mfc/test/case.py

@@ -145,7 +145,7 @@ def run(self, targets: List[Union[str, MFCTarget]], gpus: Set[int]) -> subproces
 
     def run_restart(self, targets, gpus):
         """Run a restart roundtrip: simulate to midpoint, then restart to end."""
-        mid_step = self.params['t_step_stop'] // 2
+        mid_step = (self.params['t_step_start'] + self.params['t_step_stop']) // 2
         orig = dict(self.params)
 
         try: