[geaflow/ai-memory] Define provenance model for facts, chunks, and evidence

Priority: P0
Difficulty: Starter

Context: Citation-aware answers and deletion propagation require source lineage. Existing subgraph verbalization does not provide claim-level provenance.

Scope:

- Add `ProvenanceRef` and `SourceSpan` contracts.
- Link extracted facts to document, chunk, extractor version, and schema version.
- Add JSON examples.

Constraints:

- Do not implement answer generation.
- Do not store full raw private text inside provenance objects.

Acceptance Criteria:

- Every extracted fact fixture can point back to source document and chunk.
- Missing provenance fails validation for extracted facts.

Suggested paths:

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

Start in geaflow-ai/src/main/java/org/apache/geaflow/ai/provenance and review the existing extracted-fact fixtures and subgraph verbalization context. Define the ProvenanceRef and SourceSpan contracts, add JSON examples, and verify that each fact links to its document and chunk while missing provenance fails validation.

Written by the indexing model from the issue text.