feat(arap): write off a bad debt in the engine (P4b S7)

An invoice that will not be collected comes off the receivables and is
deducted, the direct write-off method the owner chose (A-5; IRC
§166(a); Treas. Reg. §1.166-1(e), -2(a), -3(a); rules.md B-41).

Ledger::write_off posts one balanced entry, Dr 6990 Bad Debt Expense /
Cr 1200 Accounts Receivable, its receivable line tagged WriteOff so the
figures stay derived: what the invoice owes falls and what was written
off rises, and a reversal re-instates it without a new movement (D-49).
The amount is at most what is owed, and at most the least owed on any
day from the write-off's on: a later payment can't leave the invoice
owing less than nothing (B-38 INVARIANT 2). A partial write-off is
allowed, as §1.166-3(a) allows to the extent charged off. Cash-basis
books refuse it: an invoice never counted as income has no basis to
deduct (§1.166-1(e)). The bad_debts role (6990) is a control role, so
the account is created on demand and can be assigned like the others.

The same-year recovery D-49 records is a reversal of the write-off and
then a payment; a later year is a plain entry, and needs no movement.

Tests: half_paid_rest_written_off_is_written_off,
write_off_refused_on_cash_basis, write_off_above_outstanding_refused,
write_off_then_recovery_same_year.

crates/ledger-core/src/error.rs

@@ -778,6 +778,22 @@ pub enum LedgerError {
     #[error("{0}")]
     InvalidVoid(String),
 
+    /// A bad debt can't be written off on cash-basis books (rules.md
+    /// B-41 INVARIANT 3; IRC §166(a); Treas. Reg. §1.166-1(e)): the
+    /// invoice was never counted as income, so there is no basis to
+    /// deduct.
+    #[error(
+        "a bad debt can't be written off on cash-basis books: the invoice was never counted as \
+         income, so there is nothing to deduct (IRC §166; Treas. Reg. §1.166-1(e)); these books \
+         are on the cash basis (books.accounting_basis)"
+    )]
+    WriteOffOnCashBasis,
+
+    /// A write-off the books can't take: its amount, its day, or its
+    /// reason. The message says which, and what the invoice owes.
+    #[error("{0}")]
+    InvalidWriteOff(String),
+
     /// Invoice/bill due date is before its date.
     #[error("due date {0} is before document date")]
     DueBeforeDate(String),