Add account balance anchors

internal/app/app_test.go

@@ -60,6 +60,64 @@ func seed(t *testing.T, a *App, s State) State {
 	return result.State
 }
 
+func TestSaveAccountClearsStaleBalanceAnchorOnIdentityChange(t *testing.T) {
+	cases := []struct {
+		name   string
+		change func(*domain.Account)
+		clear  bool
+	}{
+		{
+			name: "currency",
+			change: func(account *domain.Account) {
+				account.Currency = "USD"
+			},
+			clear: true,
+		},
+		{
+			name: "external account",
+			change: func(account *domain.Account) {
+				account.ExternalAccountID = "new_uid"
+			},
+			clear: true,
+		},
+		{
+			name: "display name",
+			change: func(account *domain.Account) {
+				account.DisplayName = "Renamed"
+			},
+			clear: false,
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			a, s := testApp(t)
+			anchored := s.Data.Accounts[0]
+			anchored.ExternalAccountID = "old_uid"
+			anchored.AnchorBalance = "100.00"
+			anchored.AnchorDate = "2026-09-10"
+			var err error
+			s, err = a.Mutate(context.Background(), s.Revision, func(d *domain.Dataset) error {
+				return SaveAccount(d, anchored)
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			changed := anchored
+			tc.change(&changed)
+			s, err = a.Mutate(context.Background(), s.Revision, func(d *domain.Dataset) error {
+				return SaveAccount(d, changed)
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			got := s.Data.Accounts[0]
+			if tc.clear != (got.AnchorBalance == "" && got.AnchorDate == "") {
+				t.Fatalf("anchor after %s change: balance=%q date=%q", tc.name, got.AnchorBalance, got.AnchorDate)
+			}
+		})
+	}
+}
+
 // A released binary wrote include_amount into config.toml. Refusing it on
 // startup made every upgraded deployment crash-loop against its own settings
 // file, so a retired key must load and then disappear on the next save.