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 Superseded by `scripts/ffmpeg_tools.py bake|still`.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- `scripts/bom-gen.ts` — superseded by `scripts/bom_gen.py`.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/README.md

@@ -60,8 +60,8 @@ imported as a module. The dispatcher is a convenience, not a requirement.
 ## Hardware encoding
 
 This machine is an **AMD Strix Halo** (Radeon 8060S). There is no NVIDIA card,
-so the `nvenc` recipes in `ffmpegusage.txt` are reference only — they will not
-run here. What does work is **VA-API** via the Mesa `radeonsi` driver:
+so `nvenc` is unavailable here. What does work is **VA-API** via the Mesa
+`radeonsi` driver:
 
 | Codec | Encoder | Status |
 | --- | --- | --- |
@@ -173,8 +173,9 @@ Notes on the choices baked in:
   before it has fully downloaded.
 
 For anything these do not cover, use `--dry-run` to print the generated command,
-copy it, and edit it — the wrapper is a convenience, not a cage. The legacy
-recipes this grew out of are preserved in `ffmpegusage.txt`.
+copy it, and edit it — the wrapper is a convenience, not a cage. `hwprobe.py`
+reports what this host can actually encode, so you can tell at a glance whether a
+recipe will need a hardware-specific encoder.
 
 ---