feat(subtitle): data-driven conversion registry + SubtitleEdit 5 support

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

README.md

@@ -47,6 +47,10 @@ External tools on your `PATH` (recommended versions):
 - [Bento4](https://github.com/axiomatic-systems/Bento4) - ≥ 1.6.0-639
 - [dovi_tool](https://github.com/quietvoid/dovi_tool) - ≥ 2.1
 
+Optional:
+
+- [SubtitleEdit](https://github.com/SubtitleEdit/subtitleedit/releases) - ≥ 5.0 (`SeConv` CLI)
+
 ## License
 
 [GPL-3.0](LICENSE). Do not use unshackle for content you lack the rights to. Keep the core free and open; keep service code private. Be kind.