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/handbook/invoices-and-bills.md

@@ -0,0 +1,81 @@
+# Invoices and bills
+
+An invoice is money owed to you (Accounts Receivable, 1200); a bill is money
+you owe (Accounts Payable, 2000). Each is a document whose figures come from
+the ledger lines tagged with it: what is paid, discounted, written off and
+still owed is read from those lines, never kept beside them, so a reversal,
+a void or a bounced payment is right by construction. `books.documents`
+lists them, `books.document` shows one with its timeline, and
+`books.control_check` proves the control accounts equal their documents.
+
+## Step 4: void, write off, or reverse
+
+One of these, and only one, fits what happened:
+
+- **Void it** (`books.void_document`) when the document should never have
+  existed: a duplicate, the wrong customer or vendor, a test entry. It is
+  reversed, keeps its id (and, once numbered, its number) and reads void;
+  nothing is deleted. Refused while a payment stands on it: reverse that
+  payment first, and the refusal names it. Use a void for a mistake, never
+  for an invoice that was real.
+- **Write it off** (`books.write_off_invoice`) when the invoice was real
+  and the work or goods went out, but the customer will not or cannot pay.
+  That is a bad debt: the books post Dr Bad Debt Expense 6990 / Cr Accounts
+  Receivable for what will not be collected, and what it owes falls from
+  that day. The evidence of worthlessness — the collection letters, the
+  bankruptcy or liquidation notice, the collection agency's report — is
+  filed with the write-off in the same call (`upload_id`, or an
+  `existing_attachment_id` already filed), because a bad debt is deducted
+  on all the pertinent evidence (Treas. Reg. §1.166-2(a)). Write off all of
+  it, or part of it with `amount`: a partial write-off is deductible to the
+  extent charged off (Treas. Reg. §1.166-3(a)), and the rest stays owed and
+  in the aging. The books must be on the accrual basis: a cash-basis
+  company never counted the invoice as income, so there is no basis to
+  deduct (IRC §166; Treas. Reg. §1.166-1(e)), and the tool refuses.
+- **Reverse an entry** (`books.reverse_entry`) when a posted entry is
+  wrong: a payment against the wrong invoice, a wrong amount, a check that
+  bounced, a write-off made too soon. Posted is forever: the correction is
+  an opposite entry linked to the one it reverses, never an edit and never
+  a delete. Reversing a payment puts the invoice back to owing what it did;
+  reversing a write-off re-instates the debt.
+
+A bill is never written off: a vendor releases you with a vendor credit or
+by voiding its own invoice, and until then you owe it (ASC 405-20-40-1). If
+a vendor's bill was wrong, ask for a corrected bill or record a credit when
+one is built; void it only when it should never have been entered.
+
+## Recovering a debt written off
+
+- **The same tax year**: reverse the write-off — `books.reverse_entry` on
+  the write-off's entry. The invoice reopens, owing exactly what the
+  write-off took off; then record the payment with `books.receive_payment`
+  as usual. Nothing else is needed: within the year the deduction comes
+  back out, and the payment is a payment.
+- **A later tax year**: do not reverse the write-off. Record a plain entry
+  — `books.draft_entry`, `books.add_line` on each side, `books.post_entry`
+  — debiting the deposit account and crediting a "Bad debt recoveries"
+  revenue account (tax category other income), with a memo naming the
+  invoice. The invoice stays written off, and the recovery is income in the
+  year it arrives (IRC §111); the CPA sees it apart from the year's sales.
+
+Neither procedure invents a new kind of movement on the document: the books
+record what happened, and the invoice's timeline shows the write-off (and,
+if it was reversed, that it no longer stands).
+
+## What the CPA will check
+
+The agings tie to 1200 and 2000 at month and year end (they do by
+construction: the control check says so). Every write-off carries its
+reason and its evidence of collection efforts. Write-offs are not used in
+place of a credit memo for a valid invoice, and never to hide a mistake
+that a void would show. Whether a debt is worthless, and in which year, is
+a tax judgment that stays with the CPA: the books flag each write-off made
+this year in Needs attention, with the evidence beside it, until the year
+ends.
+
+## The evidence rule
+
+Every entry needs its receipt ({evidence_rule}). A document the books post
+themselves — an invoice, a bill, a payment, a write-off — carries its own:
+the issued copy, the vendor's bill, the payment's reference, the evidence of
+worthlessness. A reversal needs none of its own: its original carries it.

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",