Can Carbon::setTestNow() preserve microseconds when given a float timestamp?
Goal Determine whether Carbon's static test clock can freeze time with microsecond precision when the value is supplied as a float. Constraints Carbon 2.x truncates fractional seconds; Carbon 3.x claims to retain them but the rounding rule is undocumented. setTestNow() is global and process‑wide, affecting all Carbon calls, including CarbonImmutable instance
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