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

docs/requirements.md

@@ -34,10 +34,19 @@ requirement conflict, the requirement wins.
   total credits (the `balanced()` check).
 - **R-8 — Balance sheet.** Assets, liabilities, equity grouped by account
   type. The accounting equation holds exactly: assets = liabilities + equity.
-  Because period close (R-FUTURE-2) doesn't exist yet, current earnings (net
-  income to date) are folded into equity so the equation holds on a live book.
-- **R-9 — Profit and loss.** Revenue and expenses for the life of the ledger,
-  with net income. Period filtering arrives with R-FUTURE-2.
+  Current earnings (revenue minus expenses not yet closed to retained
+  earnings) are folded into equity so the equation holds on a live book.
+  `balance_sheet_as_of` gives the statement as of any date, a line per
+  account, counting every entry dated on or before it (closing entries
+  included: after a close the earnings sit in retained earnings).
+- **R-9 — Profit and loss.** Revenue and expenses with net income.
+  `profit_and_loss_for_period` scopes it to a date range (optionally one
+  class), a line per account, and **excludes closing entries**: a close moves
+  earnings into retained earnings and must not make a closed period read as
+  break-even. The cash flow statement excludes closing entries for the same
+  reason. Period P&Ls partition: split a range anywhere and the halves sum
+  to the whole, and a range's net income equals the change in current
+  earnings across it (property-tested).
 - **R-10 — Canonical snapshots.** `reports::snapshot()` renders the three
   core reports as deterministic JSON (decimal strings, sorted lines, fixed
   keys). Two engines fed the same books must produce identical output. This