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/parity.md

@@ -46,6 +46,9 @@ No other statuses.
 | Prepaid expense + deferred revenue amortization | Amortization schedules (Xero/QBO add-ons) | partial | hand-verified fixture, to the cent; no golden file yet |
 | Automated cross-implementation diff harness (GnuCash first) | — (`just oracle`) | partial | live: `tests/oracle_gnucash.rs` runs 200 random books through GnuCash 5.10 in a pinned container, 15,800 exact comparisons, weekly in CI (docs/oracle.md Layer 5); open: fixture books diffed as `snapshot()` output, and the machine-readable catalog with CI enforcing this floor |
 | Invoice sent tracking (who, how, to whom, when, original or reminder, the channel's reference) | Business → Invoices (Xero's "Sent"; QBO's email) | done | P4b S5; records, never sends (A-4); a send changes no figure; `invoice.sent` under `documents_v2` (D-52) |
+| Multi-document settlements with early-payment discounts (gross method) | Business → Invoices/Bills; Receive payment | done | P4b S6: one entry, one party, one application per document (`kernel::settle`, B-42); a discount beyond the terms needs a reason; a bill with an asset line is refused (D-49); the bill and its payment tested, replay equal to live |
+| A processor's fee on a payment received (gross presentation) | Xero/QBO Stripe & Shopify deposits | done | P4b S6: Dr the deposit the net, Dr processing fees the fee, Cr A/R the gross (A-4); `a_fee_posts_gross_to_receivables_and_net_to_the_bank` |
+| Bills due, aging detail and tie-out, aging exports | Business → Bills; Reports → A/R and A/P Aging | done | P4b S6: `books.bills_due`, `detail`, `control`, `ar_aging.csv`/`ap_aging.csv` in `books.export` and the CPA package, each with its `TOTAL` tie-out (`the_aging_csvs_tie_to_the_control_check`) |
 
 Workflow/UI parity (QuickBooks/Xero reference) gets its own catalog when the
 egui phase starts; semantics come first.
@@ -151,3 +154,12 @@ tracking also feeds Needs attention: an issued invoice nobody marked sent
 (past `invoicing.unsent_days`) and an overdue invoice not reminded within
 `invoicing.reminder_days` are one list in the viewer and in `books.start`,
 each offering `books.mark_sent` (step 4; U0b's one engine).
+
+### P4b S6 paying on terms (2026-10-27)
+
+S6 adds the three rows above: multi-document settlements with
+early-payment discounts (the gross method, A-8), the processor's fee
+posted gross (A-4), and the bills-due list with the agings' detail,
+tie-out and exports. The pre-S6 fixture's figures still hold (no figure
+moves), and every new write needs the journal feature `documents_v2`,
+which the owner's books declare at their next upgrade (D-52).