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

crates/ledger-cli/tests/spec_mcp.rs

@@ -887,6 +887,7 @@ fn happy_path(name: &str) -> Mcp {
     );
     m.ok("books.ar_aging", json!({"as_of": "2026-06-30"}));
     m.ok("books.ap_aging", json!({"as_of": "2026-06-30"}));
+    m.ok("books.bills_due", json!({"through": "2026-06-30"}));
     // P4b S1: a document voided, and the control accounts' tie-out.
     let dup = m.ok(
         "books.create_invoice",
@@ -1193,10 +1194,14 @@ fn happy_path(name: &str) -> Mcp {
         ("csv", "general_ledger"),
         ("json", "members_equity"),
         ("csv", "tax_summary"),
+        ("csv", "ar_aging"),
+        ("xlsx", "ar_aging"),
+        ("pdf", "ap_aging"),
     ] {
         m.ok(
             "books.export",
-            json!({"format": format, "report": report, "start": "2026-01-01", "end": "2026-12-31"}),
+            json!({"format": format, "report": report, "start": "2026-01-01", "end": "2026-12-31",
+                   "as_of": "2026-12-31"}),
         );
     }
     // The IIF takes as_of (every posted entry from the first through it),