RESTful API Design & Backward Compatibility Review

You are an API Architect and author of enterprise API design guidelines.

Review this proposed REST API contract / OpenAPI spec:

```json
POST /api/createOrder
Request:
{
  "user_id": "123",
  "items": [{ "id": "p1", "qty": 2 }],
  "payment": { "type": "card", "token": "tok_abc" }
}

Response (200 OK):
{
  "status": "success",
  "orderId": "ord_999",
  "total": 49.99
}
```

Domain Context & Target Consumers: Public B2B API consumed by mobile apps and third-party webhooks
Versioning Constraints: URL path versioning (/v1) with non-breaking field additions

Evaluate the API design against world-class REST standards:
1. **Resource URI Modeling & HTTP Methods**:
   - Check noun pluralization, hierarchical nesting, and correct HTTP verb usage (GET, POST, PUT, PATCH, DELETE).
2. **Backward Compatibility & Evolution**:
   - Will these changes break mobile apps or legacy integrations?
   - Are optional vs required fields handled gracefully?
3. **Idempotency & Concurrency Control**:
   - Are mutation operations idempotent with `Idempotency-Key` headers?
   - Is optimistic locking supported via `ETag` / `If-Match` headers?
4. **Pagination, Filtering, & Sorting**:
   - Is cursor-based pagination used for high-volume datasets instead of offset/limit?
   - Are filtering query parameters structured consistently?
5. **Error Format & Problem Details**:
   - Are errors compliant with RFC 7807 (Problem Details for HTTP APIs)?

Provide your output:
- 📑 **Design Critique & Breaking Change Warnings**
- 📐 **Recommended OpenAPI / JSON Schema Specification**
- 💡 **Example Request / Response Payloads with Edge Cases**