merge: P4b S7, bad debts (branch S7, 4b4fecb9)

The fourth of the paused branches: Ledger::write_off and the bad_debts
control role, invoice.written_off with its evidence filed in the same
store call, books.write_off_invoice, the write-off on the timeline and
in Needs attention, Lean writeOff_within_bounds, rules.md B-41 and the
bookspec examples. Twenty-eight files conflicted with the S4.4, S5 and
S6 merges; resolved as plan 8.0.2 says, keeping both sides' appends with
their seams restored, and: the scope table at 125; S7's law numbered 37;
B-41 placed before B-42; the handbook's step 4 and recovery procedures
folded into the one invoices-and-bills section; S7's wire tests in their
own module with the confirm S4.4's contract needs; S7's copy of the
payment steps dropped (S6 replaced them with Ledger::settle) and its
write-off journaled through S6's journal_auto_lines; in the old-books
fixture S7's hand-written write-off moved to a second invoice, since
S6's hand-written payment settles invoice 1 in full.

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

crates/ledger-core/src/error.rs

@@ -859,6 +859,21 @@ pub enum LedgerError {
     /// books' text rule). Plan P4b S5.
     #[error("{0}")]
     InvalidSend(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")]