[geaflow/ai-memory] Implement `ChunkVectorIndex`

[kitalkuyo-gita]

Priority: P0
Difficulty: Intermediate

Context: HugeGraph-AI-style RAG retrieves document chunks. geaflow-ai currently indexes GraphEntity, not document chunks.

Scope:

• Add chunk vector index built on VectorStore.
• Store chunk_id, source span, embedding, model metadata, and text hash.
• Add tests for insert, query, delete marker, restart, and source span return.

Constraints:

• Do not call remote embedding services in tests.
• Use deterministic fake embeddings.
• Do not return raw full document when only source span is needed.

Acceptance Criteria:

• Query vector returns chunk hits with score and source_ref.
• Deleted chunks do not appear after searchable watermark.

Suggested paths:

• geaflow-ai/src/main/java/org/apache/geaflow/ai/index/chunk