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.
PresetManager.load already heals a forward-incompatible presets.json: the
block just above repairs the legacy `custom` shape and re-saves the file.
But if the file exists and is missing a whole built-in preset (e.g. an older
install written before `reason` existed), load returned it as-is, so that
built-in stayed permanently absent — silently missing from the picker that
GET /api/presets feeds, with no way for the user to get it back.
Extend the same self-heal: after the legacy migration, fill in any built-in
presets the loaded file is missing, defaults-first so user edits win, and
persist the result. This never clobbers an intentional removal — there is no
delete path for the built-in keys (only user_templates entries can be
deleted), and presets are hidden via an `enabled: False` flag, not removal.
Checks: python -m pytest tests/test_preset_fill_missing_defaults.py (3 passed;
2 fail on the pre-fix code), the existing preset cases in
tests/test_review_regressions.py still pass, python -m py_compile
src/preset_manager.py, git diff --check.
Mazen Tamer Salah