First launch with blur blurs and saves to ~/.cache/moonset/. Subsequent
starts load the cached PNG directly (~9x faster). Cache invalidates
when wallpaper path, size, mtime, or sigma changes.
Add CHANGELOG documenting all changes since 0.1.0 and the initial
release. Add DECISIONS.md as an architectural decision log. Update
CLAUDE.md to reflect current architecture. Bump to 0.1.1 for the
security and correctness fixes in the previous commit.
