feat: period-scoped P&L and as-of balance sheet; closes don't zero periods

The engine only had life-of-ledger P&L and balance sheet totals, so every
export labeled with a date range or as-of date (PDF, XLSX, CPA package)
printed all-time numbers under that label. Core now has:

- profit_and_loss_for_period(start, end, class?) and
  balance_sheet_as_of(as_of), each with a line per account
  (StatementLine: code, name, normal-side amount) and exact totals.
- Ledger::closing_entry_ids(). Period statements exclude closing entries.
  A close (Dr revenue / Cr expense / net to Retained Earnings, dated at
  period end) moves earnings; it doesn't earn or spend. Counting it made a
  closed period's P&L read as break-even, and made the cash flow show zero
  net income plus a fake "owner contribution" to Retained Earnings. The
  as-of balance sheet keeps closing entries: after a close the earnings
  belong in Retained Earnings, not current earnings.

Tests: tests/statements.rs, a two-month fixture with every expected number
computed by hand (Jan NI 1,700; Feb 880; balance sheets on four dates; a
closed January still reporting 1,700 and a cash flow with one financing
line). Mutation-checked: dropping the closing exclusion fails it (0 vs
1,700). Properties: period P&Ls partition a year at any split, and a
range's net income equals the change in current earnings across it, with
A = L + E on both dates.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MtYEaR6M6Ybdhm6X98BWNu

crates/ledger-core/tests/statements.rs

@@ -0,0 +1,223 @@
+//! Period-scoped financial statements: profit and loss for a date range and
+//! the balance sheet as of a date, line per account.
+//!
+//! Every expected number below is computed by hand from the entries, not
+//! read off the engine. The fixture is two months of a small company:
+//!
+//! | date   | entry                         | Dr            | Cr            |
+//! |--------|-------------------------------|---------------|---------------|
+//! | Jan 2  | owner contributes 10,000      | 1000 Cash     | 3000 Capital  |
+//! | Jan 15 | client pays 2,500             | 1000 Cash     | 4000 Revenue  |
+//! | Jan 20 | rent 800                      | 6100 Rent     | 1000 Cash     |
+//! | Feb 3  | software 120 on the card      | 6000 Software | 2000 Card     |
+//! | Feb 10 | client pays 1,000             | 1000 Cash     | 4000 Revenue  |
+//! | Feb 25 | pay the card 120              | 2000 Card     | 1000 Cash     |
+
+use chrono::NaiveDate;
+use ledger_core::reports::{
+    balance_sheet, balance_sheet_as_of, profit_and_loss, profit_and_loss_for_period,
+};
+use ledger_core::{
+    AccountId, AccountType, CashFlowParams, ClassId, Currency, Ledger, Money, Side, StatementLine,
+    cash_flow,
+};
+use rust_decimal::Decimal;
+
+fn d(m: u32, day: u32) -> NaiveDate {
+    NaiveDate::from_ymd_opt(2026, m, day).unwrap()
+}
+
+fn usd(n: i64) -> Money {
+    Money::new(Decimal::from(n), Currency::USD)
+}
+
+fn dec(n: i64) -> Decimal {
+    Decimal::from(n)
+}
+
+struct Books {
+    ledger: Ledger,
+    cash: AccountId,
+    software_class: ClassId,
+}
+
+fn post(ledger: &mut Ledger, date: NaiveDate, desc: &str, dr: AccountId, cr: AccountId, n: i64) {
+    let e = ledger.create_entry(date, desc);
+    ledger.add_line(e, dr, Side::Debit, usd(n)).unwrap();
+    ledger.add_line(e, cr, Side::Credit, usd(n)).unwrap();
+    ledger.post_entry(e).unwrap();
+}
+
+/// The fixture. With `close_january`, January is closed before any February
+/// entry exists (the close sweeps every revenue and expense balance).
+fn books(close_january: bool) -> Books {
+    let mut l = Ledger::new(Currency::USD);
+    let cash = l
+        .add_account("1000", "Cash", AccountType::Asset, None, false)
+        .unwrap();
+    let card = l
+        .add_account("2000", "Card", AccountType::Liability, None, false)
+        .unwrap();
+    let capital = l
+        .add_account("3000", "Capital", AccountType::Equity, None, false)
+        .unwrap();
+    let revenue = l
+        .add_account("4000", "Revenue", AccountType::Revenue, None, false)
+        .unwrap();
+    let software = l
+        .add_account("6000", "Software", AccountType::Expense, None, false)
+        .unwrap();
+    let rent = l
+        .add_account("6100", "Rent", AccountType::Expense, None, false)
+        .unwrap();
+    let software_class = l.add_class("Apps").unwrap();
+
+    post(&mut l, d(1, 2), "Owner contribution", cash, capital, 10_000);
+    post(&mut l, d(1, 15), "Client payment", cash, revenue, 2_500);
+    post(&mut l, d(1, 20), "Rent", rent, cash, 800);
+    if close_january {
+        let jan = l.create_period("2026-01", d(1, 1), d(1, 31)).unwrap();
+        l.close_period(jan).unwrap();
+    }
+    // Software on the card, tagged to the Apps class.
+    let e = l.create_entry(d(2, 3), "Software");
+    l.add_line(e, software, Side::Debit, usd(120)).unwrap();
+    l.add_line(e, card, Side::Credit, usd(120)).unwrap();
+    l.set_line_class(e, 0, Some(software_class)).unwrap();
+    l.post_entry(e).unwrap();
+    post(&mut l, d(2, 10), "Client payment", cash, revenue, 1_000);
+    post(&mut l, d(2, 25), "Pay card", card, cash, 120);
+    Books {
+        ledger: l,
+        cash,
+        software_class,
+    }
+}
+
+fn line(code: &str, name: &str, n: i64) -> StatementLine {
+    StatementLine {
+        code: code.to_string(),
+        name: name.to_string(),
+        amount: dec(n),
+    }
+}
+
+#[test]
+fn profit_and_loss_is_scoped_to_the_period() {
+    let b = books(false);
+    let jan = profit_and_loss_for_period(&b.ledger, d(1, 1), d(1, 31), None).unwrap();
+    assert_eq!(jan.revenue, vec![line("4000", "Revenue", 2_500)]);
+    assert_eq!(jan.expenses, vec![line("6100", "Rent", 800)]);
+    assert_eq!(jan.net_income(), dec(1_700));
+
+    let feb = profit_and_loss_for_period(&b.ledger, d(2, 1), d(2, 28), None).unwrap();
+    assert_eq!(feb.revenue, vec![line("4000", "Revenue", 1_000)]);
+    assert_eq!(feb.expenses, vec![line("6000", "Software", 120)]);
+    assert_eq!(feb.net_income(), dec(880));
+
+    // Both months: 3,500 revenue, 920 expenses, lines sorted by code.
+    let both = profit_and_loss_for_period(&b.ledger, d(1, 1), d(2, 28), None).unwrap();
+    assert_eq!(
+        both.expenses,
+        vec![line("6000", "Software", 120), line("6100", "Rent", 800)]
+    );
+    assert_eq!(
+        (both.total_revenue, both.total_expenses),
+        (dec(3_500), dec(920))
+    );
+    // ... which is the life-of-ledger P&L, because nothing is outside it.
+    let all = profit_and_loss(&b.ledger).unwrap();
+    assert_eq!((all.revenue, all.expenses), (dec(3_500), dec(920)));
+
+    // One class: only the tagged software line.
+    let apps =
+        profit_and_loss_for_period(&b.ledger, d(1, 1), d(12, 31), Some(b.software_class)).unwrap();
+    assert!(apps.revenue.is_empty());
+    assert_eq!(apps.expenses, vec![line("6000", "Software", 120)]);
+
+    // A backwards range is rejected, not silently empty.
+    assert!(profit_and_loss_for_period(&b.ledger, d(2, 1), d(1, 1), None).is_err());
+}
+
+#[test]
+fn balance_sheet_is_as_of_the_date() {
+    let b = books(false);
+    // Before anything happened: empty and balanced.
+    let start = balance_sheet_as_of(&b.ledger, d(1, 1)).unwrap();
+    assert!(start.assets.is_empty() && start.equity.is_empty());
+    assert!(start.balanced());
+
+    // Jan 31: cash 10,000 + 2,500 − 800 = 11,700; equity 10,000 + 1,700.
+    let jan = balance_sheet_as_of(&b.ledger, d(1, 31)).unwrap();
+    assert_eq!(jan.assets, vec![line("1000", "Cash", 11_700)]);
+    assert!(jan.liabilities.is_empty());
+    assert_eq!(jan.equity, vec![line("3000", "Capital", 10_000)]);
+    assert_eq!(jan.current_earnings, dec(1_700));
+    assert_eq!(jan.total_equity, dec(11_700));
+    assert!(jan.balanced());
+
+    // Feb 15: cash 12,700; the card owes 120; earnings 1,700 + 880.
+    let mid = balance_sheet_as_of(&b.ledger, d(2, 15)).unwrap();
+    assert_eq!(mid.total_assets, dec(12_700));
+    assert_eq!(mid.liabilities, vec![line("2000", "Card", 120)]);
+    assert_eq!(mid.current_earnings, dec(2_580));
+    assert_eq!(mid.total_liabilities + mid.total_equity, dec(12_700));
+
+    // Feb 28: card paid off, so it drops off the statement.
+    let end = balance_sheet_as_of(&b.ledger, d(2, 28)).unwrap();
+    assert_eq!(end.total_assets, dec(12_580));
+    assert!(end.liabilities.is_empty());
+    assert!(end.balanced());
+    // ... and agrees with the life-of-ledger balance sheet.
+    let all = balance_sheet(&b.ledger).unwrap();
+    assert_eq!(
+        (all.assets, all.liabilities, all.equity),
+        (end.total_assets, end.total_liabilities, end.total_equity)
+    );
+}
+
+#[test]
+fn a_closed_period_still_reports_its_earnings() {
+    let b = books(true);
+    // The close posted Dr 4000 2,500 / Cr 6100 800 / Cr 3999 1,700 on Jan
+    // 31. The January P&L must not count it: January earned 1,700.
+    let jan = profit_and_loss_for_period(&b.ledger, d(1, 1), d(1, 31), None).unwrap();
+    assert_eq!(jan.net_income(), dec(1_700));
+    assert_eq!(jan.revenue, vec![line("4000", "Revenue", 2_500)]);
+    let feb = profit_and_loss_for_period(&b.ledger, d(2, 1), d(2, 28), None).unwrap();
+    assert_eq!(feb.net_income(), dec(880));
+
+    // After the close, January's earnings sit in retained earnings, not in
+    // current earnings; February's are current.
+    let jan_bs = balance_sheet_as_of(&b.ledger, d(1, 31)).unwrap();
+    assert_eq!(
+        jan_bs.equity,
+        vec![
+            line("3000", "Capital", 10_000),
+            line("3999", "Retained Earnings", 1_700)
+        ]
+    );
+    assert_eq!(jan_bs.current_earnings, Decimal::ZERO);
+    assert!(jan_bs.balanced());
+    let feb_bs = balance_sheet_as_of(&b.ledger, d(2, 28)).unwrap();
+    assert_eq!(feb_bs.current_earnings, dec(880));
+    assert_eq!(feb_bs.total_equity, dec(12_580));
+    assert!(feb_bs.balanced());
+
+    // The cash flow for January: net income 1,700, and the only financing
+    // flow is the owner's 10,000. The close is not an owner contribution.
+    let cf = cash_flow(
+        &b.ledger,
+        &CashFlowParams {
+            start: d(1, 1),
+            end: d(1, 31),
+            cash_accounts: vec![b.cash],
+            financing_accounts: vec![],
+        },
+    )
+    .unwrap();
+    assert_eq!(cf.net_income, dec(1_700));
+    assert_eq!(cf.financing.len(), 1);
+    assert_eq!(cf.cash_from_financing, dec(10_000));
+    assert_eq!(cf.net_change_in_cash, dec(11_700));
+}