POST /v3/instruments — fx-forward-cash-settlement-instrument

Register an FX forward that settles in cash (no physical exchange of
the two currencies — only the difference in value at fixing). The
linked account receives or pays the settlement amount on the
settlement date; the system creates an
`fx-forward-cash-settlement-transaction` for that date automatically.

Request:
{
  "data": {
    "type": "fx-forward-cash-settlement-instrument",
    "attributes": {
      "agreementDate":      "string, ISO 8601 (required)",
      "fixingDate":         "string, ISO 8601 (required) — when spot vs forward rate is compared",
      "notionalAmount":     "number (required) — notional, in primary currency",
      "primaryCcyCode":     "string (required) — ISO 4217 of the notional",
      "secondaryCcyCode":   "string (required) — ISO 4217 of the settlement",
      "settlementAmount":   "number (required) — agreed settlement, in secondary currency",
      "settlementDate":     "string, ISO 8601 (required)",
      "securityIdentifier": "string (required)"
    },
    "relationships": {
      "account": { "data": { "type": "account", "id": "string (required) — settlement account" } }
    }
  }
}

Response (201 Created):
{
  "data": {
    "id": "string",
    "type": "fx-forward-cash-settlement-instrument",
    "attributes": { /* same as request */ },
    "relationships": { /* same as request */ }
  }
}