v3.1.0 Add CurlMulti — parallel cURL requests via curl_multi_*, with keyed insertion-ordered MultiResult of immutable Response objects, optional per-request onComplete callback, concurrency cap, single-use guard and opt-in throwOnError. Port of the 2.4.x feature to the PHP 8.4 line. Also clears three pre-existing PHPStan level-10 findings.