Fix Phase 1 t_step_save to use interval relative to t_step_start

Use mid_step - t_step_start instead of bare mid_step so the save
interval aligns correctly when t_step_start > 0.

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

Author: sbryngelson

toolchain/mfc/test/case.py

@@ -152,7 +152,8 @@ def run_restart(self, targets, gpus):
             self.delete_output()
 
             # Phase 1: Run to midpoint (generates restart data)
-            self.params = {**orig, 't_step_stop': mid_step, 't_step_save': mid_step}
+            self.params = {**orig, 't_step_stop': mid_step,
+                           't_step_save': mid_step - orig.get('t_step_start', 0)}
             self.create_directory()
             result1 = self.run(targets, gpus)
             if result1.returncode != 0: