Guard run_restart against degenerate t_step_stop ≈ t_step_start

When t_step_stop - t_step_start < 2, the midpoint equals t_step_start
and Phase 1 computes t_step_save = 0, violating the min: 1 constraint.
Add an early MFCException to catch this case with a clear error message.

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

Author: sbryngelson

toolchain/mfc/test/case.py

@@ -146,6 +146,12 @@ 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_start'] + self.params['t_step_stop']) // 2
+        if mid_step <= self.params['t_step_start']:
+            raise common.MFCException(
+                f"run_restart: t_step_stop ({self.params['t_step_stop']}) is too close "
+                f"to t_step_start ({self.params['t_step_start']}) for a restart roundtrip "
+                f"(need t_step_stop - t_step_start >= 2)."
+            )
         orig = dict(self.params)
 
         try: