Replace the bash update-version.sh with a python equivalent that also
rotates the changelog (previous top-level → history[0], new empty
top-level for the new VERSION) and regenerates update-json/changelog.md
from the JSON source of truth.
Add changelog-add.py for appending a bilingual entry (EN + RU) to the
upcoming version's section.
Both scripts are uv inline scripts; shared helpers live in _changelog.py.
