That was weird. MS Defender found this.
https://www.microsoft.com/en-us/security/portal/Threat/Encyclopedia/Entry.aspx?Name=Trojan:Win32/Fuery.B!cl which is a very non-specific possible threat found by heuristic analysis. Except it was found in a 2006 Delphi project executable built when I was messing around with winamp API programming. It seems very unlikely that something I had done recently had infected a random .exe in a file written and built by me many years ago on a previous machine. I then ran a full scan and that's all it's found.

So I'm inclined to treat this as a false positive but have still wiped the directory.
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