v0.1.10 — fxContext consistency across all FX handlers

Added

• fxContext is now populated consistently across all four
  FX-affected handlers, not just invoicePaymentSucceeded. The field
  was introduced in v0.1.9 but only on invoice payments; this release
  closes the consistency hole by populating it on chargeSucceeded,
  chargeRefunded, and disputeFundsWithdrawn as well.

  Per-handler semantics:

  Handler	customerAmount	settlementAmount
  chargeSucceeded	charge.amount	bt.amount
  chargeRefunded	refund.amount (per refund)	`
  disputeFundsWithdrawn	dispute.amount	actualClawback
  invoicePaymentSucceeded	invoice.amount_paid (+ pro-rated per schedule entry)	bt.amount (+ pro-rated)

  Note on dispute design: fxContext.settlementAmount is the
  actual clawback at dispute time, not the expectedClawback
  computed at the original-charge rate. fxContext describes the
  conversion that actually happened on the event; the realized FX
  gain/loss against the original is already captured by the 7000 line
  (when present). Two pieces of information, two slots.

• New src/util/fxContext.ts houses the shared buildFxContext +
  withFx helpers. Previously inlined in invoicePaymentSucceeded;
  now imported by all four FX-aware handlers.

• New fixture charge_succeeded_fx exercises the chargeSucceeded FX
  path end-to-end: USD-50 charge converted to CAD at rate 1.375 →
  CAD 68.75 entry with fxContext { USD 5000 / CAD 6875 }. Plus QBO
  and Xero exporter goldens.

Changed

• Existing FX fixtures' expected.json regenerated to include the
  new fxContext field: charge_refunded_fx,
  dispute_funds_withdrawn_fx, dispute_funds_withdrawn_fx_rate_drift.
  Exporter (.qbo.json / .xero.json) goldens unchanged — the
  exporters ignore fxContext; it's engine-output metadata only.

Compatibility

Same-currency events on all four handlers continue to omit fxContext
from their JSON output (buildFxContext returns undefined, withFx
returns the entry untouched). Every existing same-currency fixture
passes byte-identical to v0.1.9.