Take video, audio, subtitles, or chapters from other services and mux them
with the primary download. Each cross-service authenticates, licenses, and
decrypts with its own CDM, key vault, and proxy. Audio and subtitles merge
with the primary's tracks (video and chapters replace) and flow through the
normal best-per-language selection, with an id-namespace guard so a cross
track can never shadow a primary track's DRM routing.
Per-source offsets emit an mkvmerge --sync argument; episodes auto-match with
per-type --cross-*-wanted overrides. Adds unit and offline end-to-end tests
and a cross-service muxing guide.
subtitle.language_priority and audio.language_priority prepend to the CLI
languages, so the named languages sort first and nothing is filtered.
--exact-lang now applies to the sorts too, through matching_languages, which
follows the same RFC 4647 rule as Tracks.by_language and ignores invalid tags.
Document tvdb_api_key, tvdb_pin, tvdb_order and metadata_providers, the
--tvdb and --tvdb-order flags with their behaviour when an order does
not cover a whole series, the new REST API parameters, and the Matroska
TVDB2 tag prefixes.
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