Last system of page is one staff height too far from margin
While working on #281253: Staff spacer down doesn't work at page bottom, I discovered another bug in the code handling the spacing at the bottom of the page: the last staff sits too high above the margin page. Using the default untitled score and setting the "Music box bottom margin" to 0 and then enabling "View / Show Page Margins" illustrates the problem well. The south skyline for the staff should sit directly on the page margin - that is, the lowest thing below the staff should just touch the margin. But it doesn't, there is an additional 4sp gap. That's the height of the staff, which is being counted twice in the calculation of the amount of space left on the page. That's because the height of the bottom staff is actually included in System::minBottom().
Probably it needs to be fixed within System::minBottom(), but I need to be sure there is nowhere else that this is actually being depended on. There's an equally good chance the same problem in the code leads to other problems, like too many or too few systems being selected to fit on a page.