v0.1.9 — JournalEntry.fxContext for FX recognition

Added

• JournalEntry.fxContext (optional). When the source event involved
  an FX conversion (charge.currency ≠ bt.currency), entries now carry
  a structured FxContext field exposing both sides of the conversion:

  interface FxContext {
    readonly customerCurrency: string;     // uppercase ISO 4217
    readonly customerAmount: Cents;
    readonly settlementCurrency: string;
    readonly settlementAmount: Cents;
  }

  This is the engine's contribution to multi-period FX recognition.
  Ledgerly can't auto-compute month-by-month FX gain/loss on annual
  recognition schedules without external monthly rate lookups (Stripe
  only tells us the rate at the original charge moment). But it can —
  and now does — expose enough FX provenance on every recognition entry
  that a downstream tool with a home-currency rate source can compute
  the revaluation itself.

• Populated by invoicePaymentSucceeded on FX invoices:

  • Cash entry: full conversion (customerAmount = invoice
    amount_paid, settlementAmount = bt.amount).
  • Each monthly recognition entry: PRO-RATED amounts so per-month
    customer/settlement totals across the 12-entry schedule sum to the
    invoice's preTax exactly. Month 12 absorbs both customer-side AND
    settlement-side rounding remainders.

• New fixture invoice_payment_succeeded_annual_fx and its QBO + Xero
  cash and .schedule goldens. Scenario: USD-1200 invoice settled to
  CAD at rate 1.30, no tax. Cash entry: 1010 debit CAD 1513.20 / 6000 debit CAD 46.80 / 2100 credit CAD 1560.00. Each of the 12
  recognition entries: 2100 debit CAD 130 / 4000 credit CAD 130 plus
  fxContext { USD 10000 / CAD 13000 }.

Changed

• README's currency caveats updated. Multi-period FX recognition moved
  from "spec-deferred" to "engine exposes the data; downstream computes
  the revaluation." Cross-currency payouts (Stripe converting between
  settlement currencies before bank delivery) remain deferred — they
  need real fixture data on Stripe's BT shape that ledgerly doesn't
  have.

Compatibility

fxContext is omitted entirely from JournalEntry JSON when undefined
(same-currency events). All existing same-currency fixtures —
charge_succeeded_, invoice_payment_succeeded_monthly / annual /
annual_with_tax / prorated_upgrade / prorated_downgrade / with_app_fee /
with_tax, the charge_refunded_ family, dispute_* — are byte-identical
to v0.1.8.