hook_apachesolr_process_results having no impact on results
Grr. Someone outside of StackExchange suggested a few routes of debugging that would weed out issues having to do with interaction of my code with other code. One of those suggestions was to go back to a default theme and see if my change showed there. It did. I talked to my themer to see if he had any insight.

And, that's when we hit the "a-ha" moment: We were using a custom theme based off of Fusion. (Incidentally, when I switched to stock Fusion, everything worked correctly.) At that point, it was just a matter of flipping through all of the "Appearance" settings to find that under search results none of the checkboxes were ticked. Guess what happened when I ticked the "snippet" checkbox?? Yeah....

Though, we did learn something good here, at least: If you create a new theme as a custom sub-theme of an existing theme, your subtheme does not inherit the settings of its parent theme.