POST /v3/instruments — loan-instrument

Create a fixed-rate loan instrument. POSTing a loan automatically
generates two `loan-transaction`s — one on `agreementDate` and one on
`maturityDate` — so the borrower / lender side of the cash flow lands
on the linked account.

`principalAmount` sign convention:
  positive → receivable (the customer is owed money — an asset)
  negative → payable    (the customer owes money — a liability)

Request:
{
  "data": {
    "type": "loan-instrument",
    "attributes": {
      "agreementDate":         "string, ISO 8601 (required)",
      "dayCountConvention":    "string (required) — actual/actual | 30/360 | actual/360",
      "interestRateInPercent": "number (required) — fixed rate, e.g. 1.5 = 1.5%. May be negative in periods of negative rates",
      "maturityDate":          "string, ISO 8601 (required)",
      "principalAmount":       "number (required) — denominated in account currency. Sign rules above",
      "securityIdentifier":    "string (required)"
    },
    "relationships": {
      "account": { "data": { "type": "account", "id": "string (required)" } }
    }
  }
}

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