merge: P4b S6, paying on terms (branch S6, 6c4480cd)

The third of the paused branches: Ledger::settle with discounts and the
processor's fee (kernel::settle, Kani settle_is_exact, Lean
settle_posts_a_document_step), books.receive_payment and books.pay_bill
with applications, books.bills_due, the agings' detail and tie-out, the
aging exports and the CPA package's aging CSVs, the handlers in
tools/settlements.rs. Twelve files conflicted with the S4.4 and S5
merges, resolved as plan 8.0.2 says: both routers; both test appends
(the shared match arm in arap_standing.rs spliced into both tests; the
shared `.unwrap()` in store.rs); the scope table at 124; one
invoices-and-bills handbook (S5's send step 1 with S6's steps 2, 3, 5
and procedures) registered once; docs/mcp-interface.md with S4.4's
create docs and S6's payment docs; S5's law renumbered 35 and S6's 36
(S1's is 34); the parity rows and sections of both. S6's wire tests
create their bills as S4.4's contract now requires (confirmed, the
vendor's number, a line account, the vendor's invoice uploaded) and live
in their own module beside S4.4's tests.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

docs/architecture.md

@@ -167,9 +167,12 @@ 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;
 - `sent` — `books.mark_sent`, the record that an invoice went out (P4b S5);
+- `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;