The walkthrough still described the `UNSET` sentinel and a two-error
launcher, and said nothing about the optimistic token. Brings both
documents up to what the code now does:
- partial updates use plain `None`, with the condition that makes that
safe stated explicitly so a future nullable field does not quietly
break it
- the launcher's third escape and why the failed/indeterminate line is
the #4452 guard rather than a taxonomy preference
- what `version` is for, why `save` is a conditional UPDATE rather than
read-check-write, and that it is the slice's one schema change
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>