store: voiding a fact drops its memory vectors (V-470)

Revert voided the fact row and left the vector, so recall kept serving the
voided fact's utterance and the documented repair reported success on a box that
stayed broken. There was no way to repair a poisoned box at all.

DeletePrefix covers every vector for the key, earlier rows included: their values
are superseded, and a superseded value has no business claiming a turn. It is
best-effort — the audit trail is already committed, and a fact that is voided but
still recallable beats a void that failed.

internal/store/voidvectors_test.go

@@ -0,0 +1,64 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"testing"
+	"time"
+)
+
+// Stage 3 of #470: reverting a fact reported success and left the vector that
+// was answering questions, so the documented repair did not repair.
+func TestVoidLatestFactDropsMemoryVectors(t *testing.T) {
+	ctx := context.Background()
+	s := newTestStore(t)
+	now := time.Now()
+	mem := s.VectorMemory()
+
+	if _, err := s.WriteFact(ctx, now, KindSelf, "go_version", `"1.20"`, "tap:voice", 1.0, sql.NullInt64{}); err != nil {
+		t.Fatalf("WriteFact: %v", err)
+	}
+	// The id shape actionFact writes: fact:<key>:<unix>.
+	if err := mem.Insert(ctx, "fact:go_version:1", []float32{1, 0, 0}, map[string]string{
+		"type": "fact", "text": "какая последняя версия языка Go?",
+	}); err != nil {
+		t.Fatalf("Insert: %v", err)
+	}
+	// A vector for another key must survive the void.
+	if err := mem.Insert(ctx, "fact:water:1", []float32{0, 1, 0}, map[string]string{
+		"type": "fact", "text": "запиши что я пил воду",
+	}); err != nil {
+		t.Fatalf("Insert: %v", err)
+	}
+
+	if _, _, err := s.VoidLatestFact(ctx, "go_version", "feedback", now.Add(time.Minute)); err != nil {
+		t.Fatalf("VoidLatestFact: %v", err)
+	}
+
+	got, err := mem.ByPrefix(ctx, "fact:")
+	if err != nil {
+		t.Fatalf("ByPrefix: %v", err)
+	}
+	if len(got) != 1 || got[0].ID != "fact:water:1" {
+		t.Fatalf("after the void the index holds %+v; want only fact:water:1", got)
+	}
+}
+
+func TestDeletePrefixDoesNotWidenOnWildcards(t *testing.T) {
+	ctx := context.Background()
+	s := newTestStore(t)
+	mem := s.VectorMemory()
+
+	for _, id := range []string{"fact:a_b:1", "fact:axb:1"} {
+		if err := mem.Insert(ctx, id, []float32{1, 0}, map[string]string{"type": "fact"}); err != nil {
+			t.Fatalf("Insert %q: %v", id, err)
+		}
+	}
+	n, err := mem.DeletePrefix(ctx, "fact:a_b:")
+	if err != nil {
+		t.Fatalf("DeletePrefix: %v", err)
+	}
+	if n != 1 {
+		t.Fatalf("deleted %d rows; the _ in the key must not match x", n)
+	}
+}