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-store/tests/store.rs

@@ -1875,6 +1875,8 @@ fn books_written_under_older_rules_still_open() {
         (4, "4000", "Revenue", "Revenue"),
         (5, "2000", "Accounts Payable", "Liability"),
         (6, "6500", "Supplies", "Expense"),
+        (7, "4100", "Refunds and Allowances", "Revenue"),
+        (8, "1020", "Bank", "Asset"),
     ] {
         w.append(
             "account.added",
@@ -1966,13 +1968,55 @@ fn books_written_under_older_rules_still_open() {
                "to": ["billing@fernhill.example"], "kind": "original", "reference": "msg-1"}),
     )
     .unwrap();
+    // P4b S6's new shape, hand-written as this version writes it: a
+    // payment event naming its line and the discount line it also tags.
+    // This reader tags both; the version before it would drop the discount
+    // and read the invoice paid above its total.
+    w.append(
+        "entry.created",
+        json!({"id": 4, "date": "2026-04-05", "description": "Payment on invoice 1"}),
+    )
+    .unwrap();
+    for (account, side, amount) in [
+        (3, "credit", "98.99"),
+        (3, "credit", "1.00"),
+        (7, "debit", "1.00"),
+        (8, "debit", "98.99"),
+    ] {
+        w.append(
+            "entry.line_added",
+            json!({"id": 4, "account": account, "side": side, "amount": amount,
+                   "currency": "USD"}),
+        )
+        .unwrap();
+    }
+    w.append(
+        "entry.posted",
+        json!({"id": 4, "date": "2026-04-05", "description": "Payment on invoice 1",
+        "reversal_of": null,
+        "lines": [
+            {"account": 3, "side": "credit", "amount": "98.99", "currency": "USD"},
+            {"account": 3, "side": "credit", "amount": "1.00", "currency": "USD"},
+            {"account": 7, "side": "debit", "amount": "1.00", "currency": "USD"},
+            {"account": 8, "side": "debit", "amount": "98.99", "currency": "USD"}
+        ]}),
+    )
+    .unwrap();
+    w.append(
+        "invoice.payment_received",
+        json!({"invoice_id": 1, "date": "2026-04-05", "amount": "98.99",
+               "cash_account": 8, "entry_id": 4, "line": 0,
+               "discount": "1.00", "discount_line": 1,
+               "discount_reason": "the customer paid the rest late"}),
+    )
+    .unwrap();
     drop(w);
     // Opening rebuilds from the journal; it must accept all of it.
     let _ = std::fs::remove_file(dir.join(ledger_store::db_file()));
     let store = Store::open(&dir).expect("older books still open");
     store.verify().unwrap();
     assert_eq!(store.journal_format().format, 1);
-    assert_eq!(store.ledger().chart().len(), 6);
+    assert_eq!(store.ledger().chart().len(), 8);
     assert_eq!(store.ledger().list_periods().len(), 2);
     let lines = store.ledger().get_entry(1).unwrap().lines();
     assert_eq!(lines.len(), 2);
@@ -2040,6 +2084,30 @@ fn books_written_under_older_rules_still_open() {
     assert_eq!(sends[0].to, ["billing@fernhill.example"]);
     assert_eq!(sends[0].kind, ledger_core::arap::SendKind::Original);
     assert_eq!(sends[0].reference.as_deref(), Some("msg-1"));
+    // The S6 shape: the invoice reads paid and discounted, and replay
+    // tagged the lines the event names.
+    let st = store
+        .ledger()
+        .invoice_state(1, NaiveDate::from_ymd_opt(2026, 12, 31).unwrap())
+        .unwrap()
+        .unwrap();
+    assert_eq!(
+        (st.status, st.paid, st.discounted, st.outstanding),
+        (
+            ledger_core::DocStatus::Paid,
+            "98.99".parse::<Decimal>().unwrap(),
+            "1.00".parse::<Decimal>().unwrap(),
+            Decimal::ZERO
+        )
+    );
+    assert_eq!(
+        store.ledger().line_doc(4, 0).unwrap().movement,
+        ledger_core::Movement::Payment
+    );
+    assert_eq!(
+        store.ledger().line_doc(4, 1).unwrap().movement,
+        ledger_core::Movement::Discount
+    );
 }
 
 /// The books an earlier wanderbooks wrote, one directory each under