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/src/assets.rs

@@ -128,8 +128,26 @@ fn depreciation_end(asset: &Asset) -> NaiveDate {
     }
 }
 
-/// Straight-line depreciation schedule from the purchase month through the
-/// month containing `through` (clamped to the depreciation end).
+/// Straight-line depreciation schedule from the purchase month through
+/// `through` (clamped to the depreciation end): one line per month, the
+/// last prorated by days when `through` falls mid-month.
+///
+/// ```text
+/// GOAL: how much of an asset's cost has been used up, month by month.
+/// SOURCE: docs/spec/rules.md B-15 (straight line; salvage never
+///         depreciated; partial months by days); DECISION (Stephen,
+///         2026-10-13): a schedule through a mid-month date prorates that
+///         month by days, as partial first and last months do.
+/// INVARIANTS:
+///   1. A full schedule MUST sum to exactly cost - salvage, to the cent.
+///   2. Accumulated depreciation NEVER falls; book value NEVER rises and
+///      NEVER drops below salvage.
+/// SCENARIOS:
+///   1. IF $12,000 over 12 months from Jan 1, through Jun 15:
+///      - SHOULD end with June at 500.00 (15 of 30 days), accumulated
+///        5,500.00
+///      BECAUSE depreciation accrues by the day within a month.
+/// ```
 ///
 /// Partial first and last months are prorated by days held. Each month is
 /// rounded to cents via the running-total method, and the final month