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-mcp/src/tools/write_offs.rs

@@ -128,10 +128,6 @@ impl BooksServer {
                 entry_id
             }
         };
-        Ok(Json(write_off_out(
-            store.ledger(),
-            p.invoice,
-            entry_id,
-        )?))
+        Ok(Json(write_off_out(store.ledger(), p.invoice, entry_id)?))
     }
 }