.gitignore: lock out the paths evicted in 2026-04-29 history shake
A `git filter-repo` pass on 2026-04-29 dropped 10 path patterns from
history that were already deleted from HEAD (CMake/JUCE build output,
generated PDF bifolds, an old GIF, an APK release artifact). Add them
to .gitignore so they can't accidentally re-enter history via a
forgetful `git add -A` or a tool that drops artifacts in-tree. Net
reclaim from gc + filter: 1.4 GB → 845 MB (~40%).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>