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/src/lib.rs

@@ -45,6 +45,7 @@ pub use money::{Currency, Money};
 pub use payroll::{PayrollLine, PayrollRun, PayrollRunId};
 pub use periods::{Period, PeriodId, PeriodStatus};
 pub use reports::{
-    BalanceSheet, CashFlowLine, CashFlowParams, CashFlowStatement, ClassProfitAndLoss,
-    ProfitAndLoss, TrialBalance, TrialBalanceLine, cash_flow,
+    BalanceSheet, BalanceSheetStatement, CashFlowLine, CashFlowParams, CashFlowStatement,
+    ClassProfitAndLoss, ProfitAndLoss, ProfitAndLossStatement, StatementLine, TrialBalance,
+    TrialBalanceLine, balance_sheet_as_of, cash_flow, profit_and_loss_for_period,
 };