v0.1.8 — FX gain/loss on dispute withdrawals

Added

• Realized FX gain/loss is now recognized on dispute withdrawals, not
  just refunds. When the FX rate moved between the original charge and
  the dispute, the chargeDisputeFundsWithdrawn handler routes the
  rate-movement delta to account 7000 FX Gain/Loss — same accounting
  model as the refund path shipped in v0.1.7:

  • 1200 Disputes Receivable debits the original-rate settlement
    amount (dispute.amount × originalRate), so it cleanly releases the
    receivable the original chargeSucceeded booked.
  • 1010 Stripe Clearing credits the dispute-time-rate settlement
    that Stripe actually clawed back (|clawbackBt.amount| + feeTotal).
  • 6100 Payment Disputes debits the dispute fee at the dispute rate.
  • 7000 FX Gain/Loss absorbs the difference — debit on rate move
    against the operator (realized loss), credit on rate move in their
    favor (realized gain).

  See the new dispute_funds_withdrawn_fx_rate_drift fixture for a
  worked example: USD-50 charge at CAD 1.30 (CAD 65 receivable),
  disputed later at CAD 1.40 (CAD 70 clawed back) + C$20 dispute fee →
  1200 debit 65, 6100 debit 20, 7000 debit 5, 1010 credit 90.

• expand.ts now requests charge.balance_transaction on every
  dispute event (added to the existing balance_transactions
  expansion). The original charge's BT is what the handler needs to
  compute the rate Stripe used at charge time. Same one-line pattern
  that enabled the refund FX gain/loss work in v0.1.7.

• New fixture dispute_funds_withdrawn_fx_rate_drift plus QBO and
  Xero exporter goldens.

Changed

• README's currency caveats updated. Refunds and dispute
  withdrawals are now both 7000-aware. The remaining deferrals
  (multi-period recognition rate drift, cross-currency payouts) are
  documented with their genuine blockers — multi-period drift would
  need a home-currency config or month-by-month rate lookups;
  cross-currency payouts need real fixture data on Stripe's BT shape.

Compatibility

Same-currency dispute fixtures and the existing
dispute_funds_withdrawn_fx fixture (which has a string
dispute.charge) are byte-identical to v0.1.7. The handler's
graceful fallback — when charge.balance_transaction isn't expanded,
treat expectedClawback = actualClawback and skip the 7000 line —
preserves v0.1.6/v0.1.7 behavior for any caller bypassing expand.ts.
Same-currency disputes through expand.ts also produce no 7000 line
because their true rate is 1.0 and fxDelta = 0 arithmetically.