rng ranges: Use inclusive Duration ranges in several places

Many of these call sites would panic if, somehow, the upper bound was zero. In most cases it is very complicated to see if whether this could happen.

However, there is a better answer:

Durations are (conceptually) dense, so picking the closed set (which includes its boundary) rather than the open one (which doesn't) will make little practical difference.

So change four call sites to use ..= instead of just ...

Pursuant to #920 (closed)

Setting review to @nickm who I hope is familiar with most of this code, in case there is something I am missing here.