refactor: draw the src/scripts boundary, drop dead files

Make each directory answerable by path alone: src/ is what the Remotion build
compiles, scripts/ is what you run by hand.

Removed:
- `ffmpeg.sh` — never worked; every parameter was assigned `$1`, so bake/bypass
  were broken from the start. Superseded by `scripts/ffmpeg_tools.py bake|still`.
- `src/lrc2srt.ts` — the TypeScript port; `scripts/lrc2srt.py` is the script that
  was actually maintained, and it now handles the multi-timestamp cases the TS
  version did.
- `scripts/bom-gen.ts` — superseded by `scripts/bom_gen.py`.
- `scripts/ffmpegusage.txt` — legacy command notebook. Every recipe in it is now
  reproduced by `ffmpeg_tools.py`, including the `-map 0:v:0 -map 1:a:0 -c:v copy`
  mux and the FLAC `-compression_level 12` case, so it has no unique content left.

Also drops the `bom-gen` alias from the dispatcher (dash-case was TypeScript
naming) and documents the boundary in README.md and REMOTION-QUICKSTART.md.

`npm run lint` (eslint src && tsc) passes with src/ reduced to Remotion input.

scripts/tools.py

@@ -35,7 +35,6 @@ ALIASES = {
     "ffmpeg_tools": "ffmpeg",
     "ffmpeg-tools": "ffmpeg",
     "bom_gen": "bom",
-    "bom-gen": "bom",
     "lrc": "lrc2srt",
     "srt": "srt2ass",
 }