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.
One CPython interpreter caps sustained segment throughput at ~160 MB/s;
spawned children each run the existing thread-pool path on a contiguous
chunk of the batch, reaching NIC line rate on fast CDNs. Opt-in via
--download-processes (default 1), engaged only for 24+ segment batches.
Sessions are rebuilt per child from a picklable spec; RnetSession falls
back to a single process when it has no impersonate preset.
imSp4rky