feat(bookspec): the five laws as Lean theorems (P13 step 4)

The first edition's five laws (debits equal credits; the accounting
equation; the trial balance balances; net income articulates to equity;
the cash-flow sections sum to the change in cash) are theorems in
bookspec/lean, ported from verify/lean (Basic, Entry, Statements, Closing,
Ledger under namespace Bookspec) and restated by name in Laws.lean, each a
statement about every reachable state of the books. Audit.lean prints
their axioms; the checker's lean.rs test fails on a sorry, a law not
audited, or any axiom beyond propext, Quot.sound and Classical.choice.
Money is Int, the exact count of the smallest unit: every law is a law of
addition, so the model is faithful to exact decimals.

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

bookspec/lean/Audit.lean

@@ -0,0 +1,14 @@
+import Bookspec
+
+/-! Each of the five laws' axioms are printed, and must be only Lean's
+standard three (`propext`, `Quot.sound`, `Classical.choice`) or fewer. No
+`sorryAx`, nothing assumed. `bookspec/checker/tests/lean.rs` reads this
+file's output shape; run it with `lake env lean Audit.lean`. -/
+
+open Bookspec
+
+#print axioms law_debits_equal_credits
+#print axioms law_accounting_equation
+#print axioms law_trial_balance
+#print axioms law_net_income_articulates
+#print axioms law_cash_flow_is_change_in_cash