fix(health): preserve available findings when one source fails (#626)

Health sources are read independently, so one source that fails or exceeds its limit no longer makes every health read fail. A failed source is reported as unavailable, with its name and reason. A bounded source is reported as partial, with its actual census. The other sources keep reporting, and a missing source is never shown as healthy or zero. Disabled detectors are skipped and leave no stale result.

- Attention reads the coverage of the same health read. It marks health unavailable or partial, names the affected source, and keeps the working sources' current items.
- The TUI keeps current items. During an outage it re-shows earlier items that depend on the unavailable source, under the named outage. A successful partial window replaces the previous one, and recovery clears the outage.
- No limit, threshold, policy, schema or collector changed.

Known limit: earlier items are retained under the overall health dependency, not per source, so during one source's outage an item another source has since dropped can stay visible, marked as from an incomplete feed.

Refs #613.