Smart isolation: backup/restore master model before each trial

- One-time backup of model files at study start (_model_backup/)
- Restore clean state before each trial (files stay in models/, NX refs intact)
- If a trial corrupts the model, next trial starts clean
- Best of both: NX reference integrity + trial isolation

projects/hydrotech-beam/studies/01_doe_landscape/nx_interface.py

@@ -159,13 +159,26 @@ class AtomizerNXSolver:
         self.use_iteration_folders = use_iteration_folders
         self._iteration = 0
 
-        # Iteration outputs go inside the study folder, not models/
-        # study_dir = the directory where run_doe.py lives
+        # Iteration outputs go inside the study folder
         self.study_dir = Path(__file__).parent.resolve()
         self.iterations_dir = self.study_dir / "iterations"
         self.iterations_dir.mkdir(parents=True, exist_ok=True)
         logger.info("Iterations dir: %s", self.iterations_dir)
 
+        # Create one-time backup of master model files (clean state)
+        # Restored before each trial to ensure isolation
+        import shutil
+        self._backup_dir = self.study_dir / "_model_backup"
+        if not self._backup_dir.exists():
+            logger.info("Creating master model backup at %s", self._backup_dir)
+            self._backup_dir.mkdir(parents=True)
+            for ext in ("*.prt", "*.fem", "*.sim"):
+                for f in model_dir.glob(ext):
+                    shutil.copy2(f, self._backup_dir / f.name)
+            logger.info("Backed up %d model files", len(list(self._backup_dir.iterdir())))
+        else:
+            logger.info("Using existing model backup at %s", self._backup_dir)
+
         # Find the .sim file
         sim_files = list(model_dir.glob("*.sim"))
         if not sim_files:
@@ -285,7 +298,17 @@ class AtomizerNXSolver:
         iter_dir.mkdir(parents=True, exist_ok=True)
 
         try:
-            # Step 1: Solve directly on master model (no file copying)
+            # Step 0: Restore master model from backup (clean state each trial)
+            # Files stay in models/ so NX references are intact, but content is fresh
+            import shutil
+            restored = 0
+            for backup_file in self._backup_dir.iterdir():
+                dest = self.model_dir / backup_file.name
+                shutil.copy2(backup_file, dest)
+                restored += 1
+            logger.info("Restored %d model files from backup", restored)
+
+            # Step 1: Solve directly on master model
             # NX file references stay intact — expressions updated in-place by journal
             sim_file = self.sim_file
             prt_file = self.prt_file