Add account balance anchors

internal/domain/domain.go

@@ -291,6 +291,20 @@ func Validate(d Dataset) error {
 		if a.Kind != "" && a.Kind != AccountCash && a.Kind != AccountInvestment {
 			return fmt.Errorf("account %q: kind must be %q or %q", a.ID, AccountCash, AccountInvestment)
 		}
+		// An anchor is one figure and the day it was true: neither half means
+		// anything alone, and anchoring an investment account would mask an
+		// incomplete broker history instead of exposing it.
+		if (a.AnchorBalance == "") != (a.AnchorDate == "") {
+			return fmt.Errorf("account %q: an anchor needs both a balance and its date", a.ID)
+		}
+		if a.AnchorDate != "" {
+			if a.Investing() {
+				return fmt.Errorf("account %q: a balance anchor belongs to a cash account; a broker export carries its complete history", a.ID)
+			}
+			if _, err := a.AnchorBalance.Minor(); err != nil || !validDate(a.AnchorDate) {
+				return fmt.Errorf("account %q: invalid anchor balance or date", a.ID)
+			}
+		}
 		accounts[a.ID] = a
 	}
 	for _, c := range d.Categories {