[geaflow/ai-memory] Add document replay and deduplication fixtures

Priority: P1
Difficulty: Starter

Context: Graph Memory needs replayable ingestion. If the same document is imported twice, downstream graph facts and indexes should not duplicate silently.

Scope:

- Add fixtures for duplicate documents, updated documents, deleted documents, and reordered files.
- Define expected replay behavior for `source_id + content_hash`.
- Add tests that only verify replay metadata, not full extraction.

Constraints:

- No graph backend implementation in this issue.
- No deletion propagation implementation yet; only define fixture and expected state.

Acceptance Criteria:

- Fixture clearly distinguishes duplicate, update, and delete events.
- Replay metadata can be used by later Event Log and projector issues.

Suggested paths:

- `geaflow-ai/src/test/resources/document-replay`
- `docs/graphmemory-design-doc.md` or a new `docs/geaflow-ai-graph-memory-contracts.md`

Start with the existing tests and resources under geaflow-ai/src/test, then inspect geaflow-ai/src/test/resources/document-replay and docs/graphmemory-design-doc.md. Add fixtures for duplicate, updated, deleted, and reordered documents, define expected source_id + content_hash replay metadata, and verify metadata without full extraction; done means later Event Log and projector work can consume the fixtures.

Written by the indexing model from the issue text.