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/entry-a-line-amount-is-positive.toml

@@ -0,0 +1,75 @@
+id = "entry-a-line-amount-is-positive"
+title = "A negative or zero line amount is refused; the side gives the direction"
+rule = "A line's amount MUST be greater than zero, and its side, not its sign, MUST give the direction money moves; an entry with a negative or zero line MUST be refused."
+status = "CONVENTION"
+sources = [
+  "Pacioli, Summa (1494), ch. 36: debtors at the left, creditors at the right (docs/spec/sources/pacioli-1494.md)",
+  "CONVENTION, the owner, 2026-10-13 (docs/spec/rules.md B-2): no authority forbids a negative amount outright; one reading of every line is chosen",
+]
+tags = ["accrual", "book", "us", "entry"]
+
+[setup]
+currency = "USD"
+fiscal_year_start = "01-01"
+basis = "accrual"
+rounding = "half-even"
+
+[[setup.accounts]]
+code = "1000"
+name = "Cash"
+type = "asset"
+
+[[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"
+
+# A credit of minus 100.00 would be a debit of 100.00 written the other
+# way; the books keep one way to write it.
+[[events]]
+date = "2026-01-05"
+description = "A negative credit"
+refused = "negative_amount"
+lines = [
+  { account = "1000", side = "debit", amount = "100.00" },
+  { account = "4000", side = "credit", amount = "-100.00" },
+]
+
+[[events]]
+date = "2026-01-06"
+description = "A zero line"
+refused = "zero_amount"
+lines = [
+  { account = "1000", side = "debit", amount = "0.00" },
+  { account = "4000", side = "credit", amount = "0.00" },
+]
+
+# The same 100.00 written with the sides doing the work is accepted.
+[[events]]
+date = "2026-01-07"
+description = "A sale"
+lines = [
+  { account = "1000", side = "debit", amount = "100.00" },
+  { account = "4000", side = "credit", amount = "100.00" },
+]
+
+[[expect]]
+query = "trial_balance"
+as_of = "2026-12-31"
+[expect.values]
+"1000" = "100.00"
+"3000" = "0.00"
+"4000" = "-100.00"
+"6000" = "0.00"