For links I just decided to leave out. Seemed blue was enough, and especially since link interaction in Bike only happens through link button. For general formatting I left out because underline formatting seems to be falling out of favor as a standard and when I asked in my forum most users said they didn't need it. Eventually I would like to read/write Markdown from Bike and underline isn't a part of standard Markdown.
Lots of great ideas, especially the format affinity indication/control. Is that just arrow key or arrow with cmd key? If it is only the arrow key, how do you distinguish user intent to actually move the cursor vs change the affinity?
The affinity is just treated as a new character position. When caret moves through that position is doesn't "move", but instead draws tail differently. So it's just normal arrow key with one extra state to move through. That extra state only happens at formatting boundaries, so generally the cost isn't too much.