Add startOfHour() and endOfHour() methods to ChronosTime

[dereuromark]

Summary

Adds boundary methods to ChronosTime for resetting to hour boundaries:

• startOfHour(): Resets to the start of the current hour (X:00:00.000000)
• endOfHour(): Sets to the end of the current hour (X:59:59.999999)

Example Usage

$time = ChronosTime::parse('12:30:45.123456');

$time->startOfHour()->format('H:i:s.u'); // '12:00:00.000000'
$time->endOfHour()->format('H:i:s.u');   // '12:59:59.999999'

[markstory]

looks good to me.