feat(capture): two-phase classification-aware AuditSink port (#54)

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>

ingestion/internal/capture/entities.go

@@ -140,4 +140,9 @@ type CaptureReceipt struct {
 	Errors                  []ItemError     `json:"errors"`
 	EffectiveClassification string          `json:"effective_classification,omitempty"`
 	DryRun                  bool            `json:"dry_run"`
+	// AuditBuffered is true when the central audit sink was unreachable and
+	// this capture's audit record was written to the durable local buffer
+	// instead (internal/public tier). Surfaces the degraded state to the
+	// caller per §4.4.
+	AuditBuffered bool `json:"audit_buffered,omitempty"`
 }