POST /v3/instruments — fx-forward-physical-delivery-instrument

Register an FX forward that settles by physical exchange of the two
currencies. The two linked accounts are debited and credited on the
settlement date; the system creates an
`fx-forward-physical-delivery-transaction` automatically.

Request:
{
  "data": {
    "type": "fx-forward-physical-delivery-instrument",
    "attributes": {
      "agreementDate":      "string, ISO 8601 (required)",
      "fixingDate":         "string, ISO 8601 (required)",
      "notionalAmount":     "number (required) — notional, in primary-account currency",
      "settlementAmount":   "number (required) — agreed settlement, in secondary-account currency",
      "settlementDate":     "string, ISO 8601 (required)",
      "securityIdentifier": "string (required)"
    },
    "relationships": {
      "primaryAccount":   { "data": { "type": "account", "id": "string (required) — notional side" } },
      "secondaryAccount": { "data": { "type": "account", "id": "string (required) — settlement side" } }
    }
  }
}

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