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

docs/spec/rules.md

@@ -245,7 +245,8 @@ depreciation that evenly splits the depreciable amount across the useful
 life of the asset," with "(Cost – salvage value)/Useful life"; partial
 periods: "A common method is to allocate depreciation expense based on the
 number of months the asset is owned in a year." wanderbooks prorates
-partial months by days. ASC 360-10-35-4 not yet verified (see README).
+partial months by days, including a schedule through a mid-month date
+(DECISION, Stephen 2026-10-13). ASC 360-10-35-4 not yet verified (see README).
 Tax conventions (half-year, mid-month, mid-quarter): IRS Pub. 946
 (`sources/irs-depreciation.md`), for the CPA.
 
@@ -291,10 +292,16 @@ more than 90 days past due"; it is illustrative, not a rule. The FFIEC
 Call Report instructions (bank regulators, not binding here): "The past
 due status … should be determined in accordance with its contractual
 repayment terms." A document due today is not yet past due: DECISION
-(Stephen, 2026-10-13).
+(Stephen, 2026-10-13). Aging as of a date shows only documents issued by
+then, less only payments made by then; and a payment never predates its
+invoice or bill (a real prepayment is a deposit): DECISIONS (Stephen,
+2026-10-13).
 
-**Enforced:** `arap::aging_bucket`. **Checked:** `aging_boundaries`,
-`aging_is_exclusive_and_only_ages`.
+**Enforced:** `arap::aging_bucket`, `Ledger::ar_aging`/`ap_aging`,
+`receive_payment`/`pay_bill` (`PaymentBeforeDocument`). **Checked:**
+`aging_boundaries`, `aging_is_exclusive_and_only_ages`,
+`aging_as_of_a_date_sees_only_what_existed_then`,
+`a_payment_never_predates_its_document`.
 
 ## B-19 1099 reporting thresholds