The answer to this one is that there is a loss of connectivity between the receiver and the flight controller. Key pieces of information include:
Copter will not re-arm until power cycle is almost always an indication of failsafe. There is almost nothing else in cleanflight that will cause this to occur. The copter refusing to arm at all is simply an indication that the failsafe event happened before you started flying.
The copter did not have a buzzer, but if it did, we would hear the S-O-S beep pattern that indicates failsafe. This is one reason why I always put buzzers on my copters. They're invaluable for troubleshooting.
We know that the failsafe must be between the receiver and the flight controller because the OSD shows 80%+ signal strength all the way to the ground. Also, the Taranis did not give any audio warning ("RSSI Critical") or anything like that.
To fix the issue, I powered the copter on the bench. Fortunately, the F303 powers the receiver from USB, so there was no need to plug in a battery and risk the motors spinning up. I wiggled and fiddled with the receiver to see if I could reproduce the problem. Ultimately, loosening the zip tie holding the receiver to the copter was what fixed the issue.
