refactor(arap): rustfmt the unit's files, and the write-off's SOURCE line (P4b S7)

No behaviour: the files as rustfmt leaves them after the unit's edits
(the commits before this one were made before the formatter ran), and the
Ledger::write_off spec block cites A-5 alone for the direct write-off
method (A-6 is the hand-entry refusal, not this command).

crates/ledger-core/src/ledger.rs

@@ -6329,9 +6329,9 @@ impl Ledger {
     ///         §1.166-2(a) (worthlessness on all the pertinent evidence),
     ///         §1.166-3(a) (a partial write-off, to the extent charged
     ///         off); docs/spec/rules.md B-41; plan/served.md P4b S7,
-    ///         A-5 (direct write-off), A-6; ASC 326-20 noted (an allowance
-    ///         is the GAAP preference; a small company's CPA accepts the
-    ///         direct method, stated).
+    ///         A-5 (the direct write-off method); ASC 326-20 noted (an
+    ///         allowance is the GAAP preference; a small company's CPA
+    ///         accepts the direct method, stated).
     /// INVARIANTS:
     ///   1. MUST post one balanced entry: Dr the bad_debts role (6990 Bad
     ///      Debt Expense), Cr receivables, for the amount, dated the

crates/ledger-core/tests/bookspec.rs

@@ -1171,13 +1171,14 @@ impl<'a> Books<'a> {
                     .unwrap_or_else(|| panic!("{what}: event {} posted no invoice", w.invoice));
                 let amount = match &w.amount {
                     Some(a) => exact(a, what),
-                    None => self
-                        .store
-                        .ledger()
-                        .invoice_state(invoice, day())
-                        .expect("the invoice's figures")
-                        .expect("dated by its own day")
-                        .outstanding,
+                    None => {
+                        self.store
+                            .ledger()
+                            .invoice_state(invoice, day())
+                            .expect("the invoice's figures")
+                            .expect("dated by its own day")
+                            .outstanding
+                    }
                 };
                 self.store
                     .write_off_invoice(