FileFlux v0.9.0 - Stateful Pipeline Architecture

New Features:
- 4-stage stateful pipeline (Extract → Refine → Chunk → Enrich)
- IDocumentProcessor with explicit state management
- IDocumentRefiner for content refinement
- IDocumentEnricher for chunk enrichment
- DocumentGraph for inter-chunk relationships
- Factory pattern (IDocumentProcessorFactory)

Tests: 343 tests passing (41 new)