feat(bookspec): the specification text and an example for the ledger's own laws (P13 step 4)

bookspec/spec holds the first edition: the model and the five laws
(00-model.md) and one file per area, each B-rule a section with its
status, a MUST/MAY statement, sources pinned either to a vendored source
with a verbatim quotation or to an exact external paragraph, its symbolic
form, its examples and its applicability. B-39, B-40, B-43 and B-45 are
reserved sections (allocated, not yet stated in rules.md); B-44 is stated
in rules.md and now carries its bookspec id and CONVENTION status.

Eight new examples show the laws that had none, each figure worked by
hand: an unbalanced entry is refused (Pacioli ch. 36, Pub. 583), a
negative or zero line is refused, a fraction of a cent is refused (ISO
4217), a child account takes its parent's type, the trial balance and the
accounting equation on one small book (cash 6550.25 = loan 2000 + capital
5000 + (sales 750.50 - rent 1200.25)), a reversal leaves the original
standing and a second reversal is refused, a close sweeps revenue and
expenses into Retained Earnings (3999) and the closed quarter takes no
late entry, and cents add exactly. The refusal vocabulary gains
unbalanced, negative_amount, zero_amount, too_few_lines,
finer_than_minor_unit, parent_type_mismatch, already_reversed and
period_closed; a refused entry may carry a non-positive amount.

The checker's spec.rs fails the build on a rule without its section, a
statement without MUST/SHOULD/MAY, a pin that is not verbatim in its
vendored source, examples that differ from the catalogue, or more than 60
pages.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

examples/period-a-close-sweeps-the-year-into-capital.toml

@@ -0,0 +1,108 @@
+id = "period-a-close-sweeps-the-year-into-capital"
+title = "A close zeroes revenue and expenses into equity, and the closed period takes no new postings"
+rule = "A close MUST zero every revenue and expense account through its last day and credit equity with exactly the net income; a period once closed MUST take no new postings."
+status = "AUTHORITY"
+sources = [
+  "Pacioli, Summa (1494), ch. 34: \"you shall close this account into the capital account\" (docs/spec/sources/pacioli-1494.md)",
+  "OpenStax, Principles of Accounting, Vol. 1, 5.1: temporary accounts are closed at the end of each period (docs/spec/rules.md B-11)",
+  "CONVENTION (docs/spec/rules.md B-12): a closed period is locked, which follows from posted-is-forever",
+]
+tags = ["accrual", "book", "us", "period", "close"]
+
+[setup]
+currency = "USD"
+fiscal_year_start = "01-01"
+basis = "accrual"
+rounding = "half-even"
+
+[[setup.accounts]]
+code = "1000"
+name = "Cash"
+type = "asset"
+
+[[setup.accounts]]
+code = "2000"
+name = "Loan"
+type = "liability"
+
+[[setup.accounts]]
+code = "3000"
+name = "Capital"
+type = "equity"
+
+[[setup.accounts]]
+code = "4000"
+name = "Sales"
+type = "revenue"
+tax_category = "gross_receipts"
+
+[[setup.accounts]]
+code = "6000"
+name = "Rent"
+type = "expense"
+tax_category = "rent_property"
+
+[[events]]
+date = "2026-01-02"
+description = "Members put in cash"
+lines = [
+  { account = "1000", side = "debit", amount = "5000.00" },
+  { account = "3000", side = "credit", amount = "5000.00" },
+]
+
+[[events]]
+date = "2026-02-10"
+description = "A sale"
+lines = [
+  { account = "1000", side = "debit", amount = "900.00" },
+  { account = "4000", side = "credit", amount = "900.00" },
+]
+
+[[events]]
+date = "2026-03-10"
+description = "Rent"
+lines = [
+  { account = "6000", side = "debit", amount = "350.00" },
+  { account = "1000", side = "credit", amount = "350.00" },
+]
+
+# Close January 1 through March 31 into capital.
+[[events]]
+date = "2026-03-31"
+description = "Close the quarter"
+close = { start = "2026-01-01", to = "retained_earnings" }
+
+# March 31 is closed: an entry dated inside the quarter is refused.
+[[events]]
+date = "2026-03-15"
+description = "A late sale"
+refused = "period_closed"
+lines = [
+  { account = "1000", side = "debit", amount = "50.00" },
+  { account = "4000", side = "credit", amount = "50.00" },
+]
+
+# By hand: net income = 900.00 - 350.00 = 550.00. Revenue and expense are
+# zero; the 550.00 lands in Retained Earnings (3999, the account the books
+# create for the close), so equity is 5000.00 + 550.00 = 5550.00; cash is
+# 5000 + 900 - 350 = 5550.00, so assets equal equity.
+[[expect]]
+query = "balances"
+as_of = "2026-12-31"
+[expect.values]
+"1000" = "5550.00"
+"3000" = "5000.00"
+"3999" = "550.00"
+"4000" = "0.00"
+"6000" = "0.00"
+
+# The statement for the quarter excludes the close: the year's income is
+# still reported.
+[[expect]]
+query = "profit_and_loss"
+from = "2026-01-01"
+to = "2026-03-31"
+[expect.values]
+"4000" = "900.00"
+"6000" = "350.00"
+net_income = "550.00"