[geaflow/ai-memory] Implement local persistent backend prototype

Priority: P1
Difficulty: Intermediate

Context: HugeGraph Server provides persistence in the reference capability. geaflow-ai needs at least a restartable local backend for Phase 1 reference implementation.

Scope:

- Implement a simple local backend using append-only JSONL or another existing lightweight local format.
- Support restart reload, idempotent upsert, delete marker, and checksum.
- Add corruption and partial-write tests.

Constraints:

- This is not a distributed backend.
- Do not introduce large external dependencies without discussion.
- Writes must be recoverable or fail closed.

Acceptance Criteria:

- Data survives process restart in tests.
- Truncated or malformed local store fails with actionable error or quarantine.
- Idempotent replay does not duplicate graph facts.

Suggested paths:

- `geaflow-ai/src/main/java/org/apache/geaflow/ai/backend/local`

Begin in geaflow-ai/src/main/java/org/apache/geaflow/ai/backend/local and trace the backend entry point and persistence contract used by the AI memory implementation. Use the acceptance criteria as the test checklist: restart reload, idempotent replay, deletion, checksums, and actionable handling or quarantine for malformed and partial local stores.

Written by the indexing model from the issue text.