POST /v3/instruments — private-market-fund-commitment-based-instrument

Register a commitment-based private-market fund position. The nested
`fund` object describes the fund itself; `initialCommitment` records
the initial commitment amount on a specific date.

Note: `initialCommitment` is only used on POST. It does not appear in
GET responses — once the position exists, commitments are managed via
the dedicated /v3/instruments/:id/commitments endpoints. Same with
`exitDate`: it's part of the resource model but read-only; the system
sets it when the position is closed.

Request:
{
  "data": {
    "type": "private-market-fund-commitment-based-instrument",
    "attributes": {
      "customIdentifier":  "string (optional)",
      "fund": {
        "managerName":      "string (required)",
        "name":             "string (required)",
        "ccyCode":          "string (required) — ISO 4217 fund currency",
        "strategy":         "string (required) — leveraged-buyout | private-debt | venture-capital | other",
        "investmentRegion": "string (required) — global | north-america | europe | other",
        "vintageDate":      "string, ISO 8601 (required)",
        "expectedExitDate": "string, ISO 8601 (optional)"
      },
      "initialCommitment": {
        "date":  "string, ISO 8601 (required) — date of the initial commitment",
        "value": "number (required) — initial commitment in fund currency"
      }
    },
    "relationships": {
      "depository": { "data": { "type": "depository", "id": "string (required)" } }
    }
  }
}

Response (201 Created):
{
  "data": {
    "id": "string",
    "type": "private-market-fund-commitment-based-instrument",
    "attributes": {
      "customIdentifier": "string",
      "fund": { /* same as request */ },
      "exitDate": "ISO 8601 date or null"
    },
    "relationships": {
      "depository":  { "data": { "type": "depository", "id": "string" } },
      "commitments": { "data": [ { "type": "commitment", "id": "string" } ] }
    }
  }
}