v0.1.7 — account 7000 FX gain/loss on refunds

Added

• Account 7000 FX Gain/Loss is now actually posted to. Realized FX
  gain/loss is recognized on refunds when the FX rate moved between the
  original charge and the refund. The chargeRefunded handler now reads
  the original charge's balance_transaction (already expanded by the
  receiver's expand.ts), computes the rate Stripe used at charge time,
  and books each refund's three legs at the rates that match the
  underlying movement:

  • 4900 Refunds Issued debits the original-rate settlement
    amount, so it cleanly offsets the original revenue booking (revenue
    nets to zero on a full refund).
  • 2000 Sales Tax Payable debits the proportional tax portion at the
    same original-rate basis.
  • 1010 Stripe Clearing credits the actual settlement (refund-rate)
    that Stripe clawed back.
  • 7000 FX Gain/Loss absorbs the difference — debit when the rate
    moved against the operator (realized loss), credit when it moved in
    their favor (realized gain).

  See the charge_refunded_fx fixture for a worked example:
  $100 USD charge originally at rate 1.35 (CAD 135 booked), fully
  refunded later at rate 1.40 (CAD 140 clawed back), producing
  4900 debit CAD 135 / 7000 debit CAD 5 / 1010 credit CAD 140.

• New fixture charge_refunded_fx plus QBO and Xero exporter goldens.

Changed

• README's currency caveat replaced with a precise scope statement:
  refunds are recognized; multi-period recognition rate drift, disputes
  settled at a different rate than the original charge, and
  cross-currency payouts still post in BT settlement currency without
  7000 lines (those would require state across events or additional
  config).

Compatibility

Same-currency refunds are byte-identical to v0.1.6: when
charge.currency == bt.currency, both rates are exactly 1.0, the
expected and actual settlements match, and no 7000 line is emitted.
All 5 existing same-currency refund fixtures pass unchanged.

Callers bypassing expand.ts (where charge.balance_transaction is a
string ID rather than an expanded object) fall back to rate-1.0 and
skip the 7000 line — same-currency behavior unchanged; FX behavior in
that path just loses the FX recognition (not worse than v0.1.6).