Abort-Safe Incremental JSONL Persistence Rule

Rule: Persist state using an append-only, fsync-backed JSONL log with atomic checkpoints.

Requirements

Write updates as single-line JSON objects (one logical mutation per line).
Append only (O_APPEND), never modify existing lines.
After each write batch, call fsync (or File::sync_data) before reporting success.
Treat a line as committed only if it ends with \n; ignore trailing partial lines on recovery.
Periodically create a checkpoint:
- Write full state to state.tmp
- fsync
- Atomic rename to state.jsonl
On startup:
- Load last checkpoint
- Replay log lines after it in order
On abort/panic/crash:
- No truncation
- Replay guarantees no data loss beyond last fsynced line

Outcome