Replace the hardcoded conversion if/elif in Subtitle.convert with a capability-matrix backend registry (subtitle_convert.py): each backend declares the source->target pairs it supports plus a rank, and run_conversion tries them in order as a real fallback chain. conversion_method pins a backend but still falls back (pin-then-fallback).
- Detect the cross-platform SubtitleEdit 5+ CLI (seconv) and use its --flag syntax for convert, SDH stripping, and reverse-RTL
- Protect styled ASS/SSA from automatic SRT downconversion; honor an explicit --sub-format / sidecar_format
- Read segmented fVTT (wvtt) and fTTML (stpp) directly from fragmented MP4
- Improve ASS/SSA font detection: inline \fn overrides, Format-located Fontname column, @-prefix strip, case-insensitive de-dup; covers SSA too
- Update SUBTITLE_CONFIG.md, example yaml, README; add regression tests and a backend benchmark script
