zxing has an online decoder -- https://zxing.org/w/decode.jspx -- but I've seen it fail so often that I'm wondering whether it's very "strict" / doesn't account for image distortion that much?

Or does the web site use hardcoded settings that are more tweakable in the library?

Huh, interesting -- zxing is in "Maintenance Mode Only": https://github.com/zxing/zxing

I'd be suuuuper interested to know: what's the state of the art in terms of QR decoding, both in terms of decoding speed, amenability to various image distortions, and (which I've always dreamed of) the ability to partially decode a QR code, even if the error correction fails for some parts of it, or even totally? i.e. a very very verbose debugging/decoding mode?

If you just want effective and free and don’t care about open, Google’s is the best https://developers.google.com/ml-kit/vision/barcode-scanning

Also depending on your application you could buy a hardware scanner like https://www.zebra.com/us/en/products/scanners/fixed-mount-sc... for cheap secondhand since they are all over retail. The protocol can be as simple as it “types” the barcode over USB HID like a keyboard, or there are more specialized modes.

Google's barcode kit seems to be available only for Android and iOS.

The zxing C++ port worked for me recently:

https://github.com/nu-book/zxing-cpp/