> I've heard many folks (American and otherwise) pronounce "Jack" many times in my life, and the range of utterances very comfortably includes Pépin's own "Jacques".

Then you have absolutely no ear for this kind of thing, unless you're including non-native speakers, which would be dishonest at best.

There’s no single way “native” speakers say any word, and the fact that you think there is shows you have had no exposure to the massive diversity of American accents.

What dialect of American English treats /ʒ/ and /dʒ/ as allophones at the beginning of a word (or any other context)? You've already (weirdly) accused me of being a prescriptivist, here's your chance to counter with some descriptive evidence of the kind of variation you are talking about.

I'd love to be wrong, because I'd learn a new thing. Please, educate me.

Listen, you’ll hear.

Nothing quite as weird as someone accusing me of not knowing how people in my own country say my own name.

The amount of misplaced confidence in this form is just absurd sometimes.

I'm also from the US and I've spent quite a lot of time listening to people speaking, specifically listening for how they realize phonemes. Your experience doesn't at all align with mine, either what I heard or what I've read about dialect variation in the US.

Anyway, sorry to have upset you.