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.

docs/architecture.md

@@ -147,8 +147,11 @@ each declared with `#[path]`, opening with `use super::*`, and holding a
 BooksServer`; `server.rs`'s `tool_router` adds every group's router. The
 groups are:
 
-- `documents` — invoices, bills, payments, voids, `books.documents`,
-  `books.document`, the control check and the agings;
+- `documents` — invoices, bills, voids, `books.documents`,
+  `books.document` and the control check;
+- `settlements` — payments that settle one or several documents
+  (`books.receive_payment`, `books.pay_bill`), `books.bills_due`, and the
+  agings (`books.ar_aging`, `books.ap_aging`) with their tie-out (P4b S6);
 - `entries` — `books.draft_entry`, `books.add_line`, `books.post_entry`,
   `books.reverse_entry`, `books.get_entry`, `books.search_entries`, the
   bank CSV import;