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.
sort_subtitles applied the language sort before the type sort. Python sorts
are stable, so the last pass wins and type became the major key: every forced
track, then every normal, then every SDH, each block ordered by language. In a
title with many languages this put a track far from its own SDH version, for
example Finnish at track 5 and Finnish SDH at track 42.
Add a group_by parameter that selects the major key. "type" is the default and
keeps the current order byte for byte. "language" swaps the two passes so each
language stays next to its own variants. Read it from the new subtitle.group_by
config key.
Also correct the docs: type_priority was described as sorting the variants
within each language, which was never true. It ranks the types across all
languages.
Merge cues at the string level instead of round-tripping through
pycaption, which dropped inline tags and cue settings. STYLE and
REGION blocks are kept, duplicate boundary cues splice correctly,
and a bad timing line now skips the cue instead of crashing.
Also rank pysubs2 above SubtitleEdit for ASS to TTML/VTT since
SubtitleEdit flattens inline styling there. Documented the measured
backend fidelity table and added regression tests for the merge.
imSp4rky