feat(arap): a write-off on the timeline, and the CPA's note (P4b S7)

The viewer's invoice page already drew a WriteOff movement; now the
fixture proves it: a written-off invoice reads Written off, owes nothing,
and its timeline's "Written off" row opens the entry (the screenshot
invoice-written-off).

Needs attention gains the one item the tax rule asks for: a write-off
made this calendar year is listed, one per invoice, as "Write-off
awaiting the CPA's note", with what was written off, the day, and the
prompt that opens the document. Whether a debt is worthless, and in
which year, is a tax judgment that stays with the CPA (plan 8.3.16); the
books flag it and keep the evidence beside it until the year ends. The
item is computed in ledger-package's one attention function, so the
viewer and books.start see it alike; view_model.rs compares the two reads
on the same books.

The handbook's invoices-and-bills section starts here (S4.5 adds the
issue, terms and payment steps around it): step 4 says when to void, to
write off, or to reverse, and the recovery procedures D-49 records — the
same year, reverse the write-off and record the payment; a later year, a
plain entry to a bad-debt-recoveries revenue account, the invoice staying
written off (IRC §111).

Tests: a_write_off_shows_on_the_timeline,
a_write_off_is_flagged_for_the_cpa.

crates/ledger-mcp/src/handbook.rs

@@ -73,6 +73,13 @@ pub const SECTIONS: &[(&str, &str)] = &[
         "revenue-and-payouts",
         include_str!("../handbook/revenue-and-payouts.md"),
     ),
+    // S4.5 gives invoices and bills their first three and fifth steps; S7
+    // adds step 4 (void, write off or reverse) and the recovery
+    // procedures (plan 8.2).
+    (
+        "invoices-and-bills",
+        include_str!("../handbook/invoices-and-bills.md"),
+    ),
     ("projects", include_str!("../handbook/projects.md")),
     (
         "parties-and-tins",