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/ledger.rs

@@ -568,6 +568,16 @@ impl Ledger {
         self.periods.get(&id)
     }
 
+    /// Ids of every period-closing entry. Period statements (P&L, cash
+    /// flow) exclude them: a close moves earnings into retained earnings; it
+    /// earns and spends nothing, so counting it would zero out the period.
+    pub fn closing_entry_ids(&self) -> std::collections::BTreeSet<EntryId> {
+        self.periods
+            .values()
+            .filter_map(|p| p.closing_entry)
+            .collect()
+    }
+
     /// All periods, in creation order.
     pub fn list_periods(&self) -> Vec<&Period> {
         let mut ids: Vec<PeriodId> = self.periods.keys().copied().collect();