Splits the audit port into Reserve (before any write) + Record (after),
so "confidential + sink-down → refuse before any write" is literally true
even though the audit record — which lists what landed — can only be
written afterwards.
- AuditSink.Reserve(ctx, level) → AuditOutcome | error. The error path
refuses the capture before writing: confidential + central sink down,
or the all-tiers floor (nothing can record).
- AuditSink.Record(ctx, entry, outcome) persists per the reserved outcome.
- Service: I5 gate runs after the I1 gate and after the dry-run
short-circuit (dry-run never probes the sink). AuditBuffered surfaces on
the receipt. New ErrAuditUnavailable sentinel (→ HTTP 503).
The tier→behaviour decision lives in the sink impl (#54's DegradingSink),
not the service — the service just honours Reserve's verdict.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
mathiasandClaude Opus 4.8
202212e8d5