Group selected videos by (resolution, range, codec) and mux each group into one MKV; only language is collapsed, so ranges/codecs/resolutions stay in separate files. Adds --merge-video flag + muxing.merge_video config (global and per-service), docs, and tests.
BREAKING CHANGE: The 'scene_naming' config option has been removed.
Users must configure 'output_template' in unshackle.yaml with movies, series, and songs templates. See unshackle-example.yaml for examples.