Detect statistically significant darkpool/off-exchange anomalies from Quiver Tier 1 data.

Runs in ticker mode when `ticker` is provided, otherwise portfolio mode using stored holdings.

Outputs deterministic artifacts under `reports/`:
- `report.md`
- `dashboard.md`
- `assumptions.json`
- `evidence.json`
- `evidence.md`

Strict failure semantics:
- missing quiver auth/tier
- required off-exchange dataset failure
- insufficient sample/baseline support for significance testing
- artifact write failures
