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- README: watch-first feature overview, install + libmpv requirement, fork attribution. - docs/UPSTREAM.md: upstream merge procedure, expected conflict zones, list of SageTube-only modules. - docs/BUILDING.md: per-OS libmpv install, managed-binary locations, playback reliability notes, warning that inherited GitHub release workflows download appimagetool/ffmpeg without checksum verification. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>