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/service_test.go

@@ -112,11 +112,20 @@ func (p fakePolicy) Derive(t classification.Target) classification.Level {
 }
 
 type fakeAudit struct {
-	entries []AuditEntry
-	err     error
+	entries     []AuditEntry
+	err         error // Record error
+	reserveErr  error // Reserve error (refuse before write)
+	reserveMode AuditOutcome
 }
 
-func (f *fakeAudit) Record(_ context.Context, e AuditEntry) error {
+func (f *fakeAudit) Reserve(_ context.Context, _ classification.Level) (AuditOutcome, error) {
+	if f.reserveErr != nil {
+		return 0, f.reserveErr
+	}
+	return f.reserveMode, nil
+}
+
+func (f *fakeAudit) Record(_ context.Context, e AuditEntry, _ AuditOutcome) error {
 	if f.err != nil {
 		return f.err
 	}
@@ -408,3 +417,55 @@ func TestCaptureInternalViaUSNexusAllowed(t *testing.T) {
 	require.NoError(t, err)
 	assert.True(t, rec.Insights[0].OK)
 }
+
+// --- I5 audit gate (#54) ---
+
+func TestCaptureRefusesWhenAuditReserveFails(t *testing.T) {
+	// Reserve refusing (e.g. confidential + central sink down, or the floor)
+	// aborts the capture before any write.
+	b := &fakeBrain{}
+	tr := &fakeTracker{}
+	au := &fakeAudit{reserveErr: errors.New("central sink unreachable")}
+	svc := newSvc(b, tr, nil, fakePolicy{}, au)
+
+	_, err := svc.Capture(context.Background(), CaptureInput{
+		Context:  baseCtx(),
+		Insights: []Insight{{Text: "x", Wing: "hyperguild", Hall: "facts"}},
+	})
+	require.Error(t, err)
+	assert.ErrorIs(t, err, ErrAuditUnavailable)
+	assert.Empty(t, b.writes, "nothing written when audit unavailable")
+	assert.Empty(t, tr.created)
+}
+
+func TestCaptureFlagsLocallyBufferedAudit(t *testing.T) {
+	// Reserve returns AuditBuffered (internal/public, central down) → capture
+	// proceeds and the receipt flags the degraded audit state.
+	b := &fakeBrain{}
+	au := &fakeAudit{reserveMode: AuditBuffered}
+	svc := newSvc(b, &fakeTracker{}, nil, fakePolicy{}, au)
+
+	rec, err := svc.Capture(context.Background(), CaptureInput{
+		Context:  baseCtx(),
+		Insights: []Insight{{Text: "x", Wing: "hyperguild", Hall: "facts"}},
+	})
+	require.NoError(t, err)
+	assert.True(t, rec.Insights[0].OK, "capture proceeds on degraded audit")
+	assert.True(t, rec.AuditBuffered, "receipt flags locally-buffered audit")
+	require.Len(t, au.entries, 1)
+}
+
+func TestCaptureDryRunSkipsAuditGate(t *testing.T) {
+	// dry_run must not even probe the audit sink (writes nothing anywhere).
+	au := &fakeAudit{reserveErr: errors.New("would refuse")}
+	svc := newSvc(&fakeBrain{}, &fakeTracker{}, nil, fakePolicy{}, au)
+
+	rec, err := svc.Capture(context.Background(), CaptureInput{
+		Context:  baseCtx(),
+		DryRun:   true,
+		Insights: []Insight{{Text: "x", Wing: "hyperguild", Hall: "facts"}},
+	})
+	require.NoError(t, err, "dry-run does not hit the audit gate")
+	assert.True(t, rec.DryRun)
+	assert.Empty(t, au.entries)
+}