Thanks for the suggestions and well wishes on minimal damage extent.
Engine off, the battery voltage is normal (12.5V) and the starter cranks, engine fires up just fine.
The voltage across the battery stays 12.5V, no change with RPM, alternator belt not slipping
I wish is was a fuse, but checked them and none are blown. Going over the fuse mapping, none would seem to be associated with the charging system.
This battery reversal event happened at a body shop and my normal mechanic has the car now.
Hopefully he can quickly sort this out so I can get it home before it starts snowing...