fix #31301: alignment of half notes

As discussed in the issue thread, there were two possible solutions.  The one I chose was to include half notes in the centering that had already been implemented for whole notes and larger values.  This was actually just a change of a single constant - the threshold beyond which centering should take place.  The other solution I considered was to remove the centering from all of these and instead go with *left* alignment for oversize notes - this is what most others do.  I left comments in the code showing how to do this - it's quite simple, actually.  So we can switch to that behavior any time we want, even make it a style option :-)