recall: a re-tapped fact drops its superseded vector (V-493)

The fact vector id carries a timestamp, so tapping the same key twice added a
row instead of replacing one and recall then scored the old value against the
current one. CorrectValue and VoidLatestFact already prune the key; an ordinary
re-tap is the third way a value is superseded and it did not.

actionFact now prunes fact:<key>: before inserting, so exactly one vector
survives per key. InMemoryStore gained the matching DeletePrefix, because a
test double that quietly kept both rows would pass a test the daemon fails.

The prune is best-effort and silent on a store that cannot do it: the fact row
is the truth, and a stale vector costs a wrong recall, not a lost fact.

cmd/mavend/factgate_test.go

@@ -167,3 +167,69 @@ func TestActionFact_AskedToRememberAComplaintStillWrites(t *testing.T) {
 		t.Fatalf("an explicit capture was refused: %v", err)
 	}
 }
+
+// A second tap of the same key supersedes the first, so recall must hold one
+// vector and it must be the new value (Vikunja #493). Before this the id
+// carried a timestamp, both rows stayed, and the superseded value went on
+// competing for the turn.
+func TestActionFact_ARetapSupersedesTheOldVector(t *testing.T) {
+	ctx := context.Background()
+	h, _ := newFactGateHandler(t, time.Now())
+
+	h.actionFact(ctx, router.Decision{
+		Intent:    router.IntentFact,
+		Utterance: "запиши что я пил воду",
+		Slots:     router.Slots{Key: "water", HasKey: true, Value: `"250мл"`},
+	})
+	// A later tap of the same key. The clock moves, so the old id and the new
+	// one differ — which is exactly what used to leave two rows behind.
+	h.now = func() time.Time { return time.Now().Add(time.Hour) }
+	h.actionFact(ctx, router.Decision{
+		Intent:    router.IntentFact,
+		Utterance: "запиши что я пил воду",
+		Slots:     router.Slots{Key: "water", HasKey: true, Value: `"500мл"`},
+	})
+
+	hits, err := h.recall.memStore.Search(ctx, mustEmbedPassage(t, h, "вода"), 5)
+	if err != nil {
+		t.Fatalf("memory search: %v", err)
+	}
+	if len(hits) != 1 {
+		t.Fatalf("want one vector for the key, got %d: %+v", len(hits), hits)
+	}
+	if got := hits[0].Meta["text"]; got != "water — 500мл" {
+		t.Errorf("indexed text = %q, want the current value", got)
+	}
+}
+
+// Another key is not this key. A prefix delete that widened would take the
+// whole index with it.
+func TestActionFact_ARetapLeavesOtherKeysAlone(t *testing.T) {
+	ctx := context.Background()
+	h, _ := newFactGateHandler(t, time.Now())
+
+	h.actionFact(ctx, router.Decision{
+		Intent:    router.IntentFact,
+		Utterance: "запиши что я обедал",
+		Slots:     router.Slots{Key: "meal", HasKey: true, Value: `"суп"`},
+	})
+	h.actionFact(ctx, router.Decision{
+		Intent:    router.IntentFact,
+		Utterance: "запиши что я пил воду",
+		Slots:     router.Slots{Key: "water", HasKey: true, Value: `"250мл"`},
+	})
+
+	hits, err := h.recall.memStore.Search(ctx, mustEmbedPassage(t, h, "обед"), 5)
+	if err != nil {
+		t.Fatalf("memory search: %v", err)
+	}
+	var found bool
+	for _, hit := range hits {
+		if hit.Meta["text"] == "meal — суп" {
+			found = true
+		}
+	}
+	if !found {
+		t.Fatalf("writing water dropped the meal vector: %+v", hits)
+	}
+}