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 instances created via constructor.
• Tests that compare timestamps formatted with the u specifier may be flaky if microseconds are lost.

Unresolved Questions

• What exact rounding or truncation does Carbon 2.x apply to microseconds when setTestNow receives a float?
• Does Carbon 3.x preserve the full microsecond value from a float input, or does it apply a different rounding strategy?
• Is there a recommended pattern to assert microsecond fidelity in automated tests across Carbon 2.x and 3.x?