feat(store): journal a write-off with its evidence (P4b S7)

A bad debt is deducted on evidence it is worthless (Treas. Reg.
§1.166-2), so the store will not journal one without it:
Store::write_off_invoice takes the evidence as bytes to file or as an
attachment already filed, and files it in the same call — a
receipt.attached on the write-off's entry and an attachment.linked to
the invoice (rules.md B-41). With no evidence it refuses, journaling
nothing.

The entry (Dr 6990 Bad Debt Expense / Cr 1200) and its evidence are
journaled after the engine's whole check and after the feature gate: the
new event type invoice.written_off needs documents_v2, so an older
version stops at the declaration rather than reading a written-off
invoice as still owed (B-27, D-52). Its payload is the invoice, the
entry, the day, the amount and the reason; replay tags the receivable
line WriteOff, and the projection keeps no written-off figure — it is
the tagged lines' sum (B-38).

`meta::invoice_written_off_of` reads and writes the payload; the replay
fuzzer draws the type.

8.0.6: (1) `books_written_under_older_rules_still_open` hand-writes the
new event and its entry through JournalWriter and still opens, with the
figures tailing; (2) a new fixture old-books/p4_s7_before was written by
main 4cad2cfc (the version before this unit) through the old-books-writer
harness — a numbered invoice with terms and snapshots, a partial payment,
a voided invoice and a paid bill — with its expected.json; this version
opens it and reports the same figures, the viewer included; (3) the
cut-over entry is in format_upgrade.rs's books_p2_served_cut_over_whole;
(4) documents_v2_is_its_own_feature is green and the event gate's arms
pass; (5) no figure moves: no golden changed (the previous version's
expected.json is the comparison); (6) merge does not need the owner's
go-ahead; the live books refuse the new write until he gives it.

Tests: write_off_waits_for_documents_v2, write_off_replays_and_projects,
write_off_without_evidence_is_refused; arap_props.rs gains WriteOff.

crates/ledger-core/src/lib.rs

@@ -76,8 +76,9 @@ pub use instruments::{Instrument, InstrumentId, InstrumentKind};
 pub use journal::{EntryId, EntryStatus, JournalEntry, JournalLine};
 pub use kernel::{EntryFlows, Flow};
 pub use ledger::{
-    ControlAccount, GENERAL_EXPENSES, Ledger, PAYABLES, PAYROLL, RECEIVABLES, RETAINED_EARNINGS,
-    REVENUE, ReconciliationReport, UnclearedLine, control_account, control_accounts,
+    BAD_DEBTS, ControlAccount, GENERAL_EXPENSES, Ledger, PAYABLES, PAYROLL, RECEIVABLES,
+    RETAINED_EARNINGS, REVENUE, ReconciliationReport, UnclearedLine, control_account,
+    control_accounts,
 };
 pub use money::{Currency, Money};
 pub use parties::{Party, PartyField, PartyId, Role, TinFacts, TinType, W4Facts, W9Facts};