feat(beliefs): represent unknown explicitly; absence of evidence is not falsity

A belief with no live evidence on either side now derives the status
'unknown' instead of sitting tentatively at its prior. Withdrawing every
piece of evidence leaves the proposition unknown, never rejected, and
introspection answers 'I do not know whether X' rather than quoting the
prior. Hypotheses keep their own status because they are proposals by
construction. Migration 0019 widens the status constraint; the docs define
every epistemic number and how absence of evidence is handled.

internal/beliefs/store_test.go

@@ -120,3 +120,44 @@ func TestBeliefLifecycleProvenanceAndContradiction(t *testing.T) {
 		t.Fatal("cross-mind belief list")
 	}
 }
+
+// TestUnknownIsNotFalse: withdrawing every piece of evidence leaves a belief
+// unknown, not tentatively held at its prior and not rejected.
+func TestUnknownIsNotFalse(t *testing.T) {
+	if Derive(Breakdown{Prior: 0.5, Confidence: 0.5}, concepts.Claim) != StatusUnknown {
+		t.Fatal("no evidence should derive unknown")
+	}
+	if Derive(Breakdown{Prior: 0.25, Confidence: 0.25}, concepts.Hypothesis) != StatusHypothesis {
+		t.Fatal("hypotheses keep their own status")
+	}
+	pool := storage.TestPool(t, migrations.FS)
+	ctx := context.Background()
+	m, _ := minds.NewStore(pool).Create(ctx, minds.CreateParams{Slug: "unk"})
+	es := evstore.NewStore(pool, evstore.NewBus(8), nil)
+	emit := func(ctx context.Context, q storage.Querier, n events.New) (events.Event, error) {
+		out, err := es.AppendIn(ctx, q, n)
+		if err != nil {
+			return events.Event{}, err
+		}
+		return out[0], nil
+	}
+	s := NewStore(pool)
+	res, err := s.Create(ctx, pool, emit, New{MindID: m.ID, Proposition: "the cache is warm", Epistemic: concepts.Claim, Source: "test",
+		Evidence: []EvidenceInput{{Direction: "for", Kind: "memory", RefID: "mem-1", Source: "user", Weight: 0.6, Credibility: 0.9}}})
+	if err != nil || res.Belief.Status == StatusUnknown {
+		t.Fatalf("create: %+v %v", res.Belief, err)
+	}
+	if _, err := s.RetractEvidence(ctx, pool, m.ID, "memory", "mem-1"); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := s.Reevaluate(ctx, pool, emit, m.ID, res.Belief.ID, "memory retracted"); err != nil {
+		t.Fatal(err)
+	}
+	b, _ := s.Get(ctx, pool, m.ID, res.Belief.ID)
+	if b.Status != StatusUnknown {
+		t.Fatalf("after retraction status=%s (%.2f); want unknown", b.Status, b.Confidence)
+	}
+	if b.Status == StatusRejected {
+		t.Fatal("absence of evidence must not read as falsity")
+	}
+}