fix(presets): persist presets atomically to avoid corruption on crash (#2169)

PresetManager.save() used a plain open("w") + json.dump, which truncates
presets.json before writing the new content. A crash, power loss, or
serialization error mid-write leaves the file truncated/empty and every
saved preset is lost.

Route the write through core.atomic_io.atomic_write_json (tmp file +
os.replace), matching how the rest of the codebase persists JSON state.
The helper is imported lazily so this module stays free of the heavy core
package import graph at module load time.

Adds tests/test_preset_atomic_save.py covering the source contract, a
failed-write leaving the existing file intact, and a round trip.

src/preset_manager.py

@@ -115,9 +115,12 @@ Use precise language. Show causal relationships explicitly. Quantify uncertainty
     def save(self, presets: Dict[str, Any]) -> bool:
         """Save presets to file"""
         try:
-            os.makedirs(os.path.dirname(self.presets_file), exist_ok=True)
-            with open(self.presets_file, 'w', encoding="utf-8") as f:
-                json.dump(presets, f, indent=2)
+            # Atomic write (tmp file + os.replace) so a crash or serialization
+            # error mid-write can't truncate presets.json and lose every saved
+            # preset. Lazy import keeps this module free of the heavy core
+            # package import graph at load time.
+            from core.atomic_io import atomic_write_json
+            atomic_write_json(self.presets_file, presets, indent=2)
             self.presets = presets
             return True
         except Exception as e: