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 {

internal/domain/domain_test.go

@@ -77,6 +77,18 @@ func TestDomainRejectsBrokenReferencesAndTaxonomy(t *testing.T) {
 		{"nonleaf merchant default", func(d *Dataset) { d.Merchants[0].DefaultCategoryID = "cat_food" }},
 		{"oversized tag name", func(d *Dataset) { d.Tags[0].Name = strings.Repeat("x", 201) }},
 		{"oversized category name", func(d *Dataset) { d.Categories[2].Name = strings.Repeat("x", 201) }},
+		{"anchor balance without its date", func(d *Dataset) { d.Accounts[1].AnchorBalance = "100.00" }},
+		{"anchor date without its balance", func(d *Dataset) { d.Accounts[1].AnchorDate = "2026-01-01" }},
+		{"anchored investment account", func(d *Dataset) {
+			d.Accounts[1].Kind = AccountInvestment
+			d.Accounts[1].AnchorBalance, d.Accounts[1].AnchorDate = "100.00", "2026-01-01"
+		}},
+		{"invalid anchor date", func(d *Dataset) {
+			d.Accounts[1].AnchorBalance, d.Accounts[1].AnchorDate = "100.00", "2026-02-30"
+		}},
+		{"invalid anchor balance", func(d *Dataset) {
+			d.Accounts[1].AnchorBalance, d.Accounts[1].AnchorDate = "1e2", "2026-01-01"
+		}},
 	}
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {

internal/domain/model.go

@@ -31,6 +31,16 @@ type Account struct {
 	// broker exports no counterparty column, so deposits and withdrawals carry
 	// this IBAN instead and pair with the funding account like any transfer.
 	ReferenceIBAN string `json:"reference_iban,omitempty"`
+	// AnchorBalance is the bank's booked (CLBD) balance on AnchorDate, captured
+	// once from open banking after a sync. It fixes the start balance of a
+	// date-windowed history: the money that existed before the recorded rows is
+	// AnchorBalance less every movement booked through AnchorDate, so the
+	// account's real balance is computable without complete history. The bank's
+	// figure is stored verbatim — the start balance is derived, never stored —
+	// so importing older history later corrects the derivation by itself.
+	// Cash accounts only: a broker export carries its complete history.
+	AnchorBalance Money  `json:"anchor_balance,omitempty"`
+	AnchorDate    string `json:"anchor_date,omitempty"`
 	Active        bool   `json:"active"`
 }