Add W&B or MLflow offline export prototype

## Scope
Prototype an opt-in exporter that writes TradeArena metrics/artifacts to a local W&B or MLflow-compatible output without requiring a live service.

## Acceptance criteria
- Optional dependency or plain-file mode only.
- Default quickstart remains dependency-free.
- Exporter consumes existing trajectory JSON.

## Validation
```bash
python -m pytest tests/test_observability_export.py -q
```
