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.

fuzz/fuzz_targets/fuzz_store_replay.rs

@@ -27,7 +27,8 @@ const TYPES: &[&str] = &[
     "asset.registered", "attachment.withdrawn", "bill.created", "bill.paid", "class.added",
     "customer.added", "document.filed", "entry.created", "entry.line_added",
     "entry.line_class_set", "entry.posted", "entry.reversed", "entry.vendor_set",
-    "fx.rate_set", "invoice.created", "invoice.payment_received", "journal.tail_repaired",
+    "fx.rate_set", "invoice.created", "invoice.payment_received", "invoice.written_off",
+    "journal.tail_repaired",
     "line.cleared", "payroll.run", "period.closed", "period.created", "receipt.attached",
     "receipt.linked", "receipt.unlinked", "recurring.generated", "recurring.registered",
     "schedule.posted", "schedule.registered", "vendor.1099_set", "vendor.added",