docs: add 4 DDD domain models covering all major subsystems
Create complete Domain-Driven Design specifications for:
- Signal Processing (3 contexts: CSI Preprocessing, Feature Extraction, Motion Analysis)
- Training Pipeline (4 contexts: Dataset Management, Model Architecture, Training Orchestration, Embedding & Transfer)
- Hardware Platform (5 contexts: Sensor Node, Edge Processing, WASM Runtime, Aggregation, Provisioning)
- Sensing Server (5 contexts: CSI Ingestion, Model Management, CSI Recording, Training Pipeline, Visualization)
Update DDD index (3 → 7 models) and README docs table.
Co-Authored-By: claude-flow <ruv@ruv.net>
2ad510782e