[geaflow/ai-memory] Add `Extraction SPI` with deterministic fake extractor

Priority: P0
Difficulty: Intermediate

Context: Phase 1 needs a replaceable extraction layer. Contributors should be able to test the workflow without online LLM credentials.

Scope:

- Add `Extractor` interface.
- Add `ExtractionRequest`, `ExtractionResult`, `ExtractedVertex`, `ExtractedEdge`, `ExtractedTriple`.
- Add deterministic fake extractor driven by fixtures.

Constraints:

- Do not integrate remote LLM calls yet.
- Do not write directly to `MemoryGraph`.
- The extractor must preserve original chunk IDs and source spans.

Acceptance Criteria:

- Fake extractor converts fixture chunks into typed graph facts.
- Error, empty, and partial extraction cases are covered.
- Results include schema version, extractor version, confidence, and provenance.

Suggested paths:

- `geaflow-ai/src/main/java/org/apache/geaflow/ai/extract`
- `geaflow-ai/src/test/resources/extraction/results`

Start in geaflow-ai/src/main/java/org/apache/geaflow/ai/extract and review the requested Extractor SPI and extraction result types. Read the fixtures under geaflow-ai/src/test/resources/extraction/results, then cover error, empty, and partial cases. Done means fixture chunks become typed graph facts while preserving chunk IDs and source spans, with schema version, extractor version, confidence, and provenance.

Written by the indexing model from the issue text.