feat(mcp): payments on terms, bills due, and the agings' tie-out (P4b S6)

books.receive_payment and books.pay_bill take applications, each
document's cash with an optional discount and reason, and a customer
payment may carry the processor's fee; the result names every document it
settled and what reached the account. books.bills_due lists what to pay
and the running total by due date. The agings take detail and return the
control account's tie-out, and books.export writes ar_aging/ap_aging.

The handlers live in tools/settlements.rs (the EM's split, so S4.4's
documents.rs stays free); documents.rs keeps the documents, the void and
the control check. docs/mcp-interface.md documents every changed tool and
the new one; docs/architecture.md names the group; docs/testing.md
records the tests and P12's mutation targets.

Old names (document_id, cash_account) still work; the output keeps
remaining/status for one document. A fee's processor is named in the
description, not tagged as the entry's vendor: ledger tags are entry-wide
and tagging would move the customer's party totals and 1099-NEC.

The handbook's invoices-and-bills section gains the payment, discount,
fee and bounce-replacement procedures (S4.5 can extend it). Over the wire:
one_payment_settles_two_invoices, late_discount_needs_a_reason,
a_fee_posts_gross_to_receivables_and_net_to_the_bank,
card_paid_bill_is_not_1099_nec,
multi_document_payment_counts_once_for_1099_and_party_totals,
overpayment_refused_with_the_excess,
bills_due_orders_by_due_and_discount and the_aging_export_ties_out.

Terms cannot yet be journaled on a document (S4.3) and a bill line's
account is not an MCP field (S4.4), so bill_on_terms_with_discount,
equipment_bill_payment_is_investing_cash and the discount half of
bills_due are core tests until those land; the unit's As-built block
records this.

crates/ledger-mcp/handbook/format-upgrade.md

@@ -6,3 +6,4 @@
 - Only the owner can agree to the upgrade, with the owner passphrase: in the app, which asks each time it opens the books, or with `wanderbooks books upgrade ID` in a terminal. No tool can, and you never ask for, take or type the passphrase. Tell the human, in plain words: the books wait for their go-ahead, what was refused, and that saying no keeps the books working as they are.
 - Before it upgrades, the server makes a backup of the books, their vault and keys included, and proves it whole; a backup that can't be made or proven upgrades nothing. Then the books move to the new format in one step, on the record.
 - After the upgrade an older version of wanderbooks can't open these books; the backup made first keeps them as they were. Going back is the owner's choice, never yours.
+- This upgrade is `documents_v2`: invoice numbers, terms and line accounts (S4), sent records (S5), payments that settle several documents with early-payment discounts and a processor's fee (S6), and write-offs (S7). Until it is given, those writes are refused and everything else works as before.

crates/ledger-mcp/handbook/invoices-and-bills.md

@@ -0,0 +1,63 @@
+# Invoices and bills
+
+Invoices are what customers owe the company; bills are what it owes
+vendors. Every figure on them is read from the ledger lines tagged with
+them, so a reversal, a void or a bounced payment is right by
+construction: never edit one.
+
+## 2. Record a customer payment
+
+- Ask for the date the money arrived, where it landed, and what it paid.
+  One deposit may settle several invoices of the same customer: pass
+  `applications` with each invoice and its cash amount.
+- Record how it moved (`instrument`) and the processor's or bank's
+  reference. The books record payments; they never collect them.
+- If the customer paid more than the invoices owe, the payment is
+  refused: record the excess as a plain entry to 2300 Customer Deposits
+  (no role) and apply it when the work is invoiced (D-49, A-12).
+- A check or note marked "paid in full" of a disputed amount earns the
+  UCC §3-311 warning. Record only the payment; a lawyer or the payer's
+  agreement settles the rest.
+
+## 3. Pay a vendor bill
+
+- Ask for the date, the account it left, and which bills it pays. One
+  payment may settle several bills of one vendor (`applications`).
+- Record the method and the reference. A card payment is tagged as card
+  and stays out of 1099-NEC (D-40).
+- Use `books.bills_due` for what is due and when, with each vendor's
+  early-payment discount window.
+
+### Taking an early-payment discount
+
+- A bill on "2/10 net 30" takes 2% off if paid within ten days. Pass the
+  discount beside the cash (`discount`); the books post the gross method:
+  the bill is credited in full, the discount goes to Purchase Discounts
+  (6995), and the bank gets the rest.
+- A discount at most the terms allow and dated on or before their
+  discount date needs no reason. A larger or later one needs
+  `discount_reason` (a vendor who honours it anyway); it is journaled.
+- A bill with an asset or prepaid line refuses a discount: record the
+  bill net of the discount, or reclassify with an entry Dr 6995 Purchase
+  Discounts / Cr the asset account (D-49). Never split the discount across
+  the asset lines.
+
+## 5. A processor's fee
+
+- The books never charge or collect; a processor does, and keeps a fee.
+  Pass `fee` on `books.receive_payment` (amount, and optionally the
+  expense account and the processor's vendor).
+- The entry is gross: Dr the deposit account the net, Dr the
+  processing-fee expense the fee, Cr Accounts Receivable the gross (A-4).
+  Only cash movements count toward a 1099, so the fee is not a payment to
+  the processor.
+
+## A replacement payment arrives before a bounce is known
+
+1. Park it: a plain entry Dr the bank / Cr 2300 Customer Deposits (no
+   role), described with the customer and what it is for.
+2. When the first payment bounces, reverse it (`books.reverse_entry`)
+   dated the bounce day; it un-pays its invoices.
+3. Apply the parked money with `books.receive_payment`,
+   `deposit_to: "2300"`, dated on or after the bounce, against the
+   invoices it pays. The 2300 balance returns to zero.