Carved from FilmCraft's filmcraft-captions (crates/captions at 4bec22bd): the
file readers and writers, CEA-608/708 coding, format detection and the cue
document. Rendering and the mapping onto FilmCraft's caption tracks stay there.
- time: the crate has its own exact Tick (1/254016000000 s) and rational
FrameRate, the subset of filmcraft-time the formats use.
- TTML is read through craft-xml instead of roxmltree: no entity expansion,
nesting bounded by the reader (128 levels, as before).
- parse refuses input over 64 MiB and keeps at most 1,000,000 cues.
- MCC and EBU STL name the caller's program (WriteOptions::program).
- plain_text (inline tag stripping) moves with the SCC/MCC writers that need it.
