ZFC violation: InUse was being persisted to JSON and loaded from disk,
but Reconcile() immediately overwrites it with filesystem-derived state.
Changes:
- Mark InUse with json:"-" to exclude from serialization
- Load() now initializes InUse as empty (derived via Reconcile)
- Updated test to verify OverflowNext persists but InUse does not
Per ZFC "Discover, Don't Track", InUse should always be derived from
existing polecat directories, not tracked as separate state.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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