[geaflow/ai-memory] Add searchable watermark for graph, keyword, and vector indexes

Priority: P1
Difficulty: Intermediate

Context: Graph facts and indexes can diverge. Retrieval must know whether graph, keyword, chunk vector, and entity vector projections are searchable for a given import run.

Scope:

- Define `SearchableWatermark`.
- Add fields for accepted, validated, graphed, keyword_indexed, chunk_vector_indexed, entity_vector_indexed.
- Add tests for partial projection and lag reporting.

Constraints:

- Do not implement distributed consistency.
- Do not hide partial state; expose it in trace.

Acceptance Criteria:

- Retrieval trace can show which projections were available.
- Partial index state is visible and testable.

Suggested paths:

- `geaflow-ai/src/main/java/org/apache/geaflow/ai/workflow`

Start in geaflow-ai/src/main/java/org/apache/geaflow/ai/workflow and trace how retrieval exposes projection state. Define SearchableWatermark with the listed projection fields, then add tests covering partial projection availability and lag reporting. Done means retrieval traces show available projections and partial index state is testable without distributed consistency logic.

Written by the indexing model from the issue text.