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.
Mazen Tamer Salah