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It's got to that stage in the hardware debugging where myself and @amcewen are brushing up on our conversational usb.
0110 0110!
@huffeec @MCQN_Ltd I was thinking some more overnight about how the "invalid ping" packets were always wrong by the last bit and how it seemed strange for it to be so regular.
I think our decoding was off by 1-bit. If what we thought was the last bit of the sync byte was actually the first bit of the PID then we'd have PIDs that were always 10100101. So they're all NAKs rather than pings.
Back on our hand-decoding USB from oscilloscope traces schtick.
@huffeec pointed out that they're Start-of-Frame packets rather than NAKs. (People who build protocols sending packets least-significant-bit first are even worse than those who use little-endian!)
At least now I (think) we're getting somewhere. Looks like the ESP32C3 is ignoring the computer for some reason.
A nice end to the working year.
Today some 22 ohm 0402 resistors turned up and I replaced the incorrect ones on the USB data lines on our My Bike's Got LED prototype boards.
With those two components replaced, the ESP32C3 could see the laptop's attempts to talk to it, and replied.
I can now flash code onto it over USB! 
