added update_rule for incremental change

data_updating_rule.md

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+# 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**
+- Crash/abort-safe
+- O(1) writes
+- Deterministic recovery
+- Minimal overhead