v0.6.0

Breaking:
- `stale-if-error` no longer serves a cached copy older than a stated `since`;
  the origin's error propagates instead. A fully-stale-and-uncovered batch goes
  from exit 0 to exit 4 (opt-in paths only). ADR-0022.
- `HttpClient::fetch_until` takes a `coverage_floor` parameter.

Fixed:
- Cooldown escalation decays one rung per success instead of clearing, and
  restarts once the warm window lapses, so `retry_after_ms` stops understating
  under concurrent same-host pressure. ADR-0023.

SCHEMA_VERSION is unchanged at "2".