Suggested update to manuscript and code example

[muaawiyahtucker]

I have written a suggested improvement to the course material and placed a detailed explanation in an issue here.

Here is the suggested code improvements

[aaron-recompile]

Hi — you're technically correct on both points. The even-Y requirement does apply to the internal key P pre-tweak, not to Q, and the d' vs d notation does conflict with BIP340 convention. I'll incorporate these corrections into the manuscript directly.
However I can't merge this PR as-is because ecc.py (+322 lines) introduces a custom ECC library that doesn't fit the existing code architecture. The manuscript and key-tweaking script changes are valid — if you want to resubmit with only those two files, I'll review again.
Either way, the technical catch on even-Y is the kind of thing that makes the book better. Thank you.