fix(core): payments never predate their documents; aging as of a date; refusals leave no draft

Three spec decisions from Stephen (2026-10-13), raised by the spec-blind
test writers, each test-first:

- A payment dated before its invoice or bill is refused
  (PaymentBeforeDocument); a real prepayment is a deposit. Test:
  a_payment_never_predates_its_document. Replay is unaffected (it goes
  through replay_invoice_payment / replay_bill_payment).
- Aging as of a date shows only documents issued by then, less only the
  payments made by then. The ledger now records each document's payment
  entries (live and on replay), and aging counts those dated by as_of.
  Test: aging_as_of_a_date_sees_only_what_existed_then; the A/R and A/P
  conservation properties' aging oracles updated to the confirmed rule
  (they had assumed aging ignored dates).
- A schedule through a mid-month date prorates that month by days; the
  code was right and its doc wrong. The spec-blind test that disagreed is
  rewritten to the confirmed spec and no longer ignored.

Also found while wiring payments: a payment to an unknown cash account
created the payment's draft and then failed, leaving it behind; the cash
account is now checked before anything is created.

The unreviewed golden van_schedule.json is gone: every row was first
checked against an independent hand computation (all 61 matched), then
replaced by van_schedule_is_the_hand_computed_one, which asserts that
computation directly (548.39, 59 x 1,000.00, 451.61; total 60,000.00).

Tests: 452. No spec-disagreements left ignored.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

crates/ledger-core/tests/assets.rs

@@ -261,18 +261,39 @@ fn schedule_through_purchase_date_covers_one_day() {
 }
 
 #[test]
-fn golden_van_depreciation_schedule() {
-    // The canonical van's month-by-month schedule, pinned byte-for-value.
-    // Hand-verified: 61 months, first 548.39 = 1000 x 17/31, last 451.61 =
-    // 1000 x 14/31, lifetime total exactly 60000.00.
+fn van_schedule_is_the_hand_computed_one() {
+    // Rank 1, computed by hand (rules.md B-15), replacing an unreviewed
+    // golden file (2026-10-13; every row of the old file was checked
+    // against this computation first, and matched).
+    // $60,000 over 60 months is $1,000 a month. Bought 2026-01-15: January
+    // holds 17 of 31 days = 548.39; February 2026 through December 2030 are
+    // 59 full months; life ends 2031-01-15, so January 2031 holds 14 of 31
+    // days = 451.61. 548.39 + 59,000 + 451.61 = 60,000.00 exactly.
     let (mut ledger, cost, accum, expense) = test_ledger();
     let id = van_asset(&mut ledger, 60_000, cost, accum, expense);
     let asset = ledger.get_asset(id).unwrap();
     let sched = depreciation_schedule(asset, date(2032, 1, 1));
-    let got = serde_json::to_value(&sched).unwrap();
-    let golden: serde_json::Value =
-        serde_json::from_str(include_str!("golden/van_schedule.json")).unwrap();
-    assert_eq!(got, golden);
+    assert_eq!(sched.len(), 61);
+    let mut accumulated = Decimal::ZERO;
+    for (k, line) in sched.iter().enumerate() {
+        let (year, month) = (2026 + (k as i32) / 12, (k as u32) % 12 + 1);
+        let amount = match k {
+            0 => Decimal::new(54839, 2),
+            60 => Decimal::new(45161, 2),
+            _ => Decimal::new(100000, 2),
+        };
+        accumulated += amount;
+        assert_eq!(line.period, format!("{year}-{month:02}"));
+        assert_eq!(line.amount, amount, "{}", line.period);
+        assert_eq!(line.accumulated, accumulated, "{}", line.period);
+        assert_eq!(
+            line.book_value,
+            Decimal::from(60_000) - accumulated,
+            "{}",
+            line.period
+        );
+    }
+    assert_eq!(accumulated, Decimal::from(60_000));
 }
 
 #[test]