Simply put, this was an Ethernet cable issue between the fiber modem and router.

In more detail, I know that cable quality is an obvious consideration when it comes to supporting higher data rates. I didn't originally suspect it, though, because the cable I had between router and modem was only about 6 feet long and I had read (on the interwebs... :)) that CAT5 should work "if the cable isn't too long" so I thought my 6 foot CAT5 cable may not be the culprit. Evidently, not guaranteed. I just stuck a 24 inch cable in I happened to have handy and, boom, it negotiated a 1000mbps link. I am definitely going to make sure I upgrade to CAT5e or better where I want the > 100mbps rate.