I picked up a TOPDON BT100 battery tester. You clip it on, pick the battery type and enter the battery's cold cranking amps (CCA) rating. My labels were covered, so I had to pull the hold-down bracket off to find it: 710 CCA.
The result: 12% health and 247 of 710 cold cranking amps. That's a bad battery.
The old battery was about two and a half years old and had some leaking around it. It's Texas. These go bad.
I jumped it with my AutoWit SuperCap 2. It has saved me a few times. It charges itself up when you connect it, even from a weak battery, and you can also charge it over USB or from the cigarette lighter. I connected the pack and waited for its ready signal before starting the car.
It started on the first try, so I let it run, took it for a drive and headed to AutoZone.
Their tester said the same thing: bad battery, cranking health fail, replace now. So a Duralast Gold went in right there in the parking lot, at night, and it started right up.