Well, I just killed the process. After running 12 hours, it reached 49% but then dropped back to 0% to start all over!
I am now in the market for a less buggy product.
The process keeps running, but it is decelerating at a rate that looks like it will take eons to complete. Furthermore, it only monotonically increases its estimated time remaining, which indicates it will never complete.
I am using “CCleaner Professional v5.88.9346 (64-bit)” on a one-TB HDD.