* fix(usage): preserve unrefreshed cached quotas and filter unknown status for incidents

- Retain cached provider quotas across early partial refreshes during startup, preventing premature eviction when a subset of providers completes before checksum.
- Filter out unknown (statusKnown: false) and inactive status indicators in primaryIncidentProvider, panelToolTipText, ProviderHeader, and FullRepresentation, preventing stale outages from masking active incidents.
- Add QtTests for UsageCache.restore and notification wiring.

* test(usage): cover startup cache restoration and incident guards

Document partial startup restoration and unknown-status presentation. Preserve the existing successful refresh label across the late checksum callback, and verify the real lifecycle with the usage-retention smoke scenario.